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AO Silver Member, 155 Lounge Winner 09
7,713 Posts
Discussion Starter #1 (Edited)
[UPDATED 28 March 2012: additional info regarding Connects2 CTSAR003 steering wheel control interface]

This post will hopefully guide people wishing to install their own aftermarket headunit into their 147, GT or 156. The main considerations when installing a new headunit are:

1. ensuring the powerwires are matched correctly
2. catering for Bose amplifier where applicable
3. catering for steering wheel controls where applicable
4. fitting a dashkit fascia which surrounds the new headunit to match the shape of the dash: these ensure a cosmetically 'nice' fitment; only the series 1 (pre-update) 156 does not require a fascia

I will not explain here, the physical removal of the original headunit and fitment of the new. This is generally straight forward, and explained in many other posts.

IMPORTANT: I suggest you have at least a basic knowledge of autoelectrics and how to connect wires safely and reliably, before undertaking installation of a headunit. A MULTIMETER is an essential tool for confirming function of wires, and highly recommended to be used. I take no responsibility for incorrect wiring that subsequently leads to damage to either your headunit or car. Please proceed carefully :) .


This is the main bugbear with our cars. Despite having standardised 'ISO' plugs, the wiring pattern does not conform with the ISO standard!

There a 2 main ISO plugs (A power and B speakerwire), though joined together into a single big one, coloured black. These can be separate, and different colours in earlier Alfa's.

There are also 3 smaller 'mini-ISO' plugs (C), coloured YELLOW, GREEN, and BLUE. They are interlocked. The plugs look like this:
Alfa ISO plugs

Each wire/terminal is given a standardised/ISO label. The letter (A,B,C,etc) refers to the plug/block that the wire is fitted; the number refers to the position within that plug. This schematic shows the labelling of wires in an Alfa headunit. IMPORTANTLY, these are the positions of the terminals at the back of the headunit; correlate these CAREFULLY with the positions in the Alfa plugs; in particular, observe the wiring numbers in the C/mini-ISO plugs:
Alfa ISO plug wire labels

Other plugs you may find include: GSM (green square plug), GPS (grey square plug)

NOTE: there are subtle differences between the cars, including:

*147/GT - ISO/mini-ISO plugs are mostly the same across these models
*of note:
**these cars utilise CANBUS wiring for functions like turning the headunit ON/OFF via the ignition key, speed pulse wires, data output wires for multifunction display, dimming the backlighting when sidelights/headlights are ON (wires A1, A3)
**there is usually no ignition/switched powerwire
**there is no illumination/dimmer wire
**there are 3 steering wheel control wires (RC-A (C10), RC-B (C11), GND (C12))
*there should be an additional (non-ISO) white plug which can provide useful connections; the plug is unused, and I am unsure of its intended purpose (? possibly used with Connect Nav+ system)
Alfa GT wiring schematic

*156 facelift dash - similar to 147/GT plugs and wiring pattern but some minor differences
*of note:
**these cars utilise CANBUS (wires A1, A3)
**there is an ignition/switched powerwire (wire A4)
**there is an illumination/dimmer wire (wire A6)
**there are 3 steering wheel control wires (wires C10,11,12)
Alfa facelift 156 wiring schematic (photo courtesy of trashbat)

*156 prefacelift dash
*of note:
**there is an ignition/switched powerwire (wire A3) and two constant/BATTery wires (wires A4, A7)
**there is an illumination/dimmer wire (wire A6)
**there are no steering wheel control wires
Alfa prefacelift 156 wiring schematic

*156 factory nav headunits
*of note:
**there are 2 steering wheel control wires (RC (wire C11), GND (wire C12)) and a powerwire (RC +12V (wire C10))
Alfa 156 nav steering wheel control wires

In summary, the Alfa plugs have the following wires:

power ISO :
8-hole plug, wire positions labelled A1-A8

*communicates with ECU; functions associated with turning headunit on/off with ignition key; headunit security code; illumination backlighting
*not applicable to non-CANBUS cars like prefacelift 156

*applicable only to cars fitted with the Bose stereo
*triggers headunit into 'Bose mode', which includes 'low level' audio output, disabling some EQ/tuning functions
*activated by connecting +12V

*shares functions with CANBUS B wire
*rather than CANBUS, this wire is the ignition wire in prefacelift 156 (labelled ACC)

A4: IGN (ignition)
*'switched' powerwire: wire switches from 0V to +12V when you turn the ignition key from OFF to ON (many cars - non Alfas - have an ACC key position which also offers +12V)
*wire is ABSENT from the plug in the GT and most 147; I have seen one in the 147GTA
*present in the facelift 156
*position of this wire is not is an Alfa! Normal ISO position would be A7

A5: ANT (antenna)
*leads to the antenna booster, to boost the radio signal
*activated by a +12V output from the headunit

A6: ILLUM (illumination)
*0V with side/headlights OFF, and switches to +12V when you turn them ON
*absent in the 147 and GT, which rely on CANBUS signals to trigger illumination changes
*present in the 156

A7: Ub (battery)
*main 'constant' powerwire: wire is always live +12V, and supplies the main power to run the headunit
*also provides memory back-up of radio station presets and any EQ settings
*position of this wire is not is an Alfa! Normal ISO position would be A4

A8: GND (ground)
*leads directly to the chassis

speakerwire ISO : 8-hole plug, wire positions labelled B1-B8
*Alfa plug conforms with ISO pattern, meaning it is simply plug-and-play

B1: RR+ (rear right speaker positive)
B2: RR- (rear right speaker negative)
B3: FR+ (front right speaker positive)
B4: FR- (front right speaker negative)
B5: FL+ (front left speaker positive)
B6: FL- (front left speaker negative)
B7: RL+ (rear left speaker positive)
B8: RL- (rear left speaker negative)

YELLOW mini-ISO : 6-hole plug, wire positions labelled C1-C6
*C1-5: unused
*C6: BOOSTER (only present if Bose system fitted)
**remote output wire to trigger Bose amplifier on/off
**switches from 0V to +12V when headunit turned on, turning the amplifier on at the same time

GREEN mini-ISO : 6-hole plug, wire positions labelled C7-C12
*C7-C9: telephone wires
**includes MUTE trigger and telephone audio input
**not applicable in most cars
*C10-C12: steering wheel control wires (if fitted)
**3 wires that cater for steering wheel controls in facelift 156, 147, GT

BLUE mini-ISO : 8-hole plug, wire-positions labelled C13-C20
*CD changer connection (if fitted)

For further reference go to the .pdf file attached to TallPaul's first post in his excellent thread; includes wire descriptions for 'unused white plug' in GT:


IMPORTANT: virtually all aftermarket headunits have the wiring discussed here; however, you should always check to see it does correlate, by referring to the owner's manual for the new headunit

Most UK-spec headunits will include a wiring harness that has ISO plugs on one end (for power and speakerwires), and a plug for the back of your headunit (eg: Alpine-specific plug). If yours does not have ISO plugs, I strongly recommend you buy a harness with them. You can either buy a harness with the ISO-headunit plugs (eg: ISO-Alpine harness), or ISO plugs with bare wires that you join (eg: solder) to your headunit wires. Having ISO plugs is much easier than cutting off the Alfa plugs and joining every wire separately.

Some headunits have ISO sockets on the back, to accept the Alfa plugs directly (eg: Blaupunkt) but you still must cater for wiring alterations.

In summary, an aftermarket headunit wiring harness will have the following standard wires:

YELLOW: 'constant' +12V powerwire
*this must be connected to the A7 (Ub) wire in the Alfa ISO power plug

*in most instances, you will need to redirect this wire from the A4 position to A7; many harnesses have bullet-terminal connections along this wire to facilitate this change
*in the 147/GT, if you are using the Connects2 CTSAR003 interface you do not need to redirect the wire and can leave it in position A4; it is therefore plug-and-play

Headunit wiring harness with YELLOW/RED wires swapped to suit a facelift Alfa 156 (photos courtesy of adruss85)

RED: 'switched' +12V powerwire
*in the prefacelift 156, this can be connected to the A3 (ACC) wire in the Alfa ISO power plug
*in the facelift 156, this can be connected to the A4 (IGN) wire; many harnesses have bullet-terminal connections along this wire to facilitate the change
*in the GT and most 147, you will need to redirect this wire to a suitable switched powerwire because there is no IGN wire in the Alfa ISO plug; options for where to connect this wire include:
1. cigarette lighter powerwire: in the 147/GT, this is the 'middle of three' wires in the plug, so you can extend and join the headunit's RED wire there
2. A7 'constant' powerwire: this can provide the necessary +12V to trigger the headunit ON; but because some headunits will remain in standby even if you turn them OFF via their power button, or detach the security face, they will continue to draw a small amount of power and can flatten your battery over several days if the car is unused; I recommend adding an inline switch along the headunit's RED wire to completely disable power along the trigger wire, and therefore turn the headunit OFF completely; the switch can be mounted, hidden, under the dash within easy reach
3. in the GT (and some 147s): the 'unused white plug' behind the dash has a suitable wire in position 7
*only if you are using the Connects2 CTSAR003 interface, you do not need to redirect this wire; the interface interprets CANBUS signals and provides an ignition wire in the plug (position A4) that connects to the headunit harness; it is therefore plug-and-play

*should correctly correspond to wire A8

BLUE: ANT wire
*should correctly correspond to wire A5
*Pioneer headunits have a shared output wire (BLUE/WHITE) to power the antenna booster and trigger an external amplifier on/off; on UK-spec harnesses, there is a sidewire to connect to your amplifier, in addition to the main wire fitted to the ISO plug; most other brands have a separate ANT wire and REMote amplifier trigger wire

*when delivered +12V, this wire functions to dim the display/backlighting of your headunit, to reduce glare at night
*connect to a suitable 'switched' powerwire: 0V with side/headlights OFF, +12V with side/headlights ON
*the 156 has suitable wire in position A6 to connect to the headunit
*the 147/GT has suitable wire in the 'unused white plug' in position 8
*the cigarette lighter has a backlight powerwire; I measured in the GT this was not +12V but around +3V; however, this may be sufficient to activate the function in your headunit and no harm in trying
*only if you are using the Connects2 CTSAR003 interface, you do not need to redirect this wire; the interface interprets CANBUS signals and provides an illumination wire in the plug (position A6) that connects to the headunit harness; it is therefore plug-and-play

Your headunit may have other wires which are not essential to the normal function of the headunit, and would normally remain disconnected. However, they may still have a role in your car:

*this remote output wire delivers +12V to trigger an amplifier on
*required for Bose systems (see below)

*this mute input wire attenuates/mutes the volume when activated (grounded)
*suitable for connection to handsfree phone kits

*refer to the following thread to locate the speed pulse signal wire located in the fusebox area
*this wire may be required for GPS headunits

NOTE: the Alfa CANBUS wires remain disconnected when you install an aftermarket headunit. The exception is with aftermarket GPS headunits, where a CANBUS adaptor can be connected to the CANBUS wires for purposes of speed pulse data.

In summary, the power ISO plug simply plugs from your new headunit into the Alfa ISO plug, with the exception of swapping the YELLOW powerwire so that it connects to the Alfa A7 wire, and in the facelift 156, swapping of the RED powerwire so that it connects to the Alfa A4 wire. In the 147 and GT, you will need to direct the RED powerwire to a suitable 'switched' wire. In the prefacelift 156, the RED powerwire can connect to A3. These wire-swaps are not required if using the Connects2 CTSAR003 interface in the 147/GT.

The speakerwire ISO plug is plug-and-play. Because the Alfa plug conforms to the ISO wiring pattern, there is no need to change any wires from your new headunit. However, if you have a Bose amplifier, it is preferable to utilise the RCA preouts for audio signal instead of the speakerwires (see below).

NOTE: if fitted, Alfa CD changer is not compatible with any aftermarket headunits (EXCEPT Blaupunkt); the BLUE mini-ISO plug will be left disconnected and the CD changer unusable

NOTE: currently, no aftermarket headunit can transfer text/data to the multifunction display (MFD) because there is no interface available; you will lose all relevant info on the MFD including CD track numbers, radio station text, etc.

*the Alfa antenna cable has an ISO-style plug which simply pushes into an ISO antenna socket on an aftermarket headunit
*some headunits use a DIN-style socket; in this situation, you will require an ISO-DIN adaptor plug, which may be supplied with the headunit
ISO-DIN antenna adaptor


The Bose amplifier is triggered ON/OFF via the single wire in the YELLOW mini-ISO plug (wire C6). This wire must subsequently be connected to the REM OUTPUT wire from your new headunit (usually BLUE/WHITE), which supplies the +12V signal.

There are options to make this connection:
1. fit matching mini-ISO terminal/plug onto end of REM OUTPUT wire and connect to mini-ISO plug
2. cut off YELLOW mini-ISO plug and attach matching (bullet) terminal onto wire and connect to REM OUTPUT wire

The audio signal from the factory headunit comes via the speakerwires. In 'Bose mode', the headunit provides a 'low level' (low voltage) signal. When no Bose amplifier is fitted, the speakerwires provide a 'high level' (amplified) signal to drive your speakers directly.

To connect audio to the Bose amplifier from your new headunit, it is preferable to use the RCA preouts. This is because the signal bypasses any internal amplifier ICs, so remains of optimal quality (low distortion, higher clarity/bandwidth/signal-noise-ratio). Because the preouts use 'phono' style terminals, and because the Alfa plug is ISO style, it is recommended to use an RCA-ISO adaptor harness. You can either construct your own, or buy one like the nexxia PC2-36-AMPR.

The PC2-36-AMPR includes a mini-ISO connection for the REM OUTPUT wire. You can purchase the PC2-36-AMP alone, which does not include the remote connection.

NOTE: when utilising RCA preouts, two important conditions should apply:
1. buy a headunit with at least 2 sets of RCA preouts (4 channels); the Bose amplifier relies on 4 channels of audio input; utilise the 'front' and 'rear' preouts (not 'sub' preout)
2. buy a headunit with at least '4V preouts'; this strength of output is required to drive the Bose amplifier to high volume; if the signal is lower voltage (eg: 2V or less), full volume on the headunit may not offer a signal powerful enough to drive the amplifier to high volume

NOTE: when using the RCA preouts, your new headunit's speakerwire ISO plug remains disconnected

NOTE: if you do not have a headunit meeting the above conditions, it is recommended you utilise the 'speakerwire' outputs, which are higher voltage; this usually means simply plugging the supplied headunit's speakerwire ISO plug into the corresponding Alfa ISO plug; because the signal may be higher than the factory headunit, the volume may be very loud with only a low setting on the volume control, and each increment of change causes a relatively large change in volume; unfortunately, there is no 'happy medium' between using the low level preouts and high level speakerwire outputs in this situation; the Bose amplifier sensitivity cannot be adjusted


If your car has steering wheel controls (SWC), you will probably be inclined to buy a headunit with an input for this feature. However, if your headunit does not have this feature, interfaces are available that utilise an infrared transmitter; this can be mounted 'in line of sight' of your headunit, and signals transmitted to the remote sensor (presuming the headunit has a remote control sensor). Enquire with your ICE specialist about a suitable product.

To 'hardwire' your steering controls, you will require:
1. headunit with suitable input socket
2. specific SWC interface: this will 'translate' the Alfa signals to those compatible with your brand of headunit (eg: Alfa-Alpine SWC interface)

With the exception of some Alpine models, you will need to buy the interface separately. They are often 2-piece, comprising the actual interface box, and a wiring adaptor harness/patch lead (to connect between the interface and your headunit).

SWC examples are the Alpine products (for Alpine headunits without the inbuilt interface):
KCE-870B interface
KWE-897E adaptor harness

Connects2 is a brand offering SWC interfaces. The interface is the product CTSAR001 and CTSAR003 (includes CANBUS connections) for the facelift 156, 147, and GT. An example of the adaptor harness is CTALPINELEAD (for Alpine headunit). They should be available through various online and ICE stores.

The CTSAR003 is particularly suitable for the 147/GT (non-GTA). Similar to the CTSAR001, it plugs into the Alfa green mini-ISO plug (for steering control signals) and main power-ISO and speakerwire-ISO plugs; unlike the CTSAR001, the CTSAR003 also interprets the CANBUS signals and provides suitable outputs to go to your new headunit: ignition turn-on, illumination trigger. It repositions all wires to suit your new headunit, resulting in very simple plug-and-play connection for virtually any model headunit.

NOTE: nexxia is an online store offering Connects2 products and other fitting kits
Nexxia Car Stereo Fitting Kits

NOTE: Pioneer offer interfaces to suit their headunits, including models to suit late model Alfa Romeos; please refer to the chart for part numbers:

NOTE: Alpine have offered their own interfaces/harnesses, like those i linked photos to; their websites have no, or limited, info on these products, so i am unsure of their current availability

The interface will plug directly to the Alfa GREEN mini-ISO plug to receive signals from the Alfa steering wheel controls. Some will also require a +12V power feed, and can connect to the REM OUTPUT or ACC wires on your headunit. The adaptor harness will connect to the interface, and have an output lead that plugs into your headunit. For example, Alpine utilise a conventional 3.5mm jack.

As above, some Alpine headunits have an inbuilt SWC interface. These models have 3 wires at the back, which connect to the corresponding 3 SWC wires in the GREEN mini-ISO plug. The headunit is programmable, to learn the Alfa signals.

For 156 cars fitted with factory nav headunits, an SWC can still be connected, but first requires some modification to the factory wiring. The green mini-ISO plug has only 2 steering wheel control wires, but these arise from a nav-specific interface under the dash; the conventional 3 wires feed into this interface, and are available to utilise. Refer to this thread for details:


To conform to the shape of the dash, you will want a dashkit fascia trim. These are available for the 147, 156 'update' dash, and GT. For GTA models, anthracite versions may be on offer. There are also 2-DIN fascias to suit the 147 and GT.

Again, Connects2 offer these:

Connects 2 Fascia Plates
Alfa Car Stereo Fitting Kits

There are reports that the Autoleads versions are a better fit (for the 147/GT). Oddly, their product numbers are exactly the same as for Connects2/nexxia; same manufacturer?


The colour of the fascia may be a poor match for your dash. It may be necessary to respray the fascia with paint colour-matched to your dash. Refer to a paint specialist for advice; many retailers will offer a service to colour-match paint, then provide this in an aerosol can.


I hope this will prove helpful to people. If you find any mistakes or have suggestions for changes, please PM me, and I will make adjustments accordingly. Note that i am not a professional installer, nor work in the ICE industry. ICE is merely another passion and hobby of mine, though I have had many years of experience with custom installations into my own Alfa Romeos, with top level success in car audio competition. I live in Australia, but have made every effort to make reference to UK products.

Credit is given to those AO members who have provided photos, references, and advice. :thumbs:


0 Posts
Great Shiny :thumbs: :thumbs:

This will deffo come in handy for when I get to fitting a new head unit and all :)

13,001 Posts
Very Informed and well written shiney...


13 Posts
Great Post - very helpful.

My concern is the interface with the dash display... Is this preserved with the an aftermarket unit?


147 JTDm 16v

AO Silver Member, 155 Lounge Winner 09
7,713 Posts
Discussion Starter #8
NOTE: currently, no aftermarket headunit can transfer text/information to the multifunction display (MFD) because there is no interface available; you will lose all relevant info on the MFD including CD track numbers, radio station text, etc.
^^^ in amongst the information overload. ;)

answer is: no.


2,429 Posts
Great stuff Shiny.

Re. the single DIN fascia adaptor for the GT, avoid Connects2 and use the Autoleads product (in my experience the Connects2 is an awful fit and colour).

1 Posts
This is awesome. I've been searching the net for a month now, looking for some info - now I have it. Thank you, Shiny, you rock!
Going to go tinker with the old Alpine now, wish me luck!
Hopefully the steering wheel controls will work now...

LE: I'm saving this one to print and store next to my Alfa Manual.

AO Silver Member, 155 Lounge Winner 09
7,713 Posts
Discussion Starter #12
no probs. if you find any discrepancies, let me know.


36 Posts
hi i am having some difficulty in finding out why my stereo comes on but theres no sound? it goes through the bose amp - is there a wire that needs to be connected to my stereo differently in order to make it work ? would it be easier to buy the origininal alfa stereo rather than trying to get a jvc one to work? .

29 Posts
i have the same problem with a sony head unit but it is all explained above, its having the patience to do it!!!

great thread shiny well done, it explains every question i have about installing my new head unit.

my next question is CAN YOU COME AND FIT IT FOR ME lol

i have the bose system, what head unit do you recomend as this sony i just bought only has 2 phono's on it, i couldnt power it up so i took an ignition live from the fag lighter and i used the white and red wire from the fag lighter, i did this before i read this thread so then i couldnt work out why none of my speakers were working!

so in short what do you recomend to do for the easiest option?
buy head unit with 4 phono's?


AO Silver Member, 155 Lounge Winner 09
7,713 Posts
Discussion Starter #15
this probably isn't the right thread to be asking the questions. maybe start new threads if i haven't answered adequately. :)

hi i am having some difficulty in finding out why my stereo comes on but theres no sound? it goes through the bose amp - is there a wire that needs to be connected to my stereo differently in order to make it work ? would it be easier to buy the origininal alfa stereo rather than trying to get a jvc one to work? .
well, yes, the Bose requires the remote trigger wire. the jvc should have a 'remote output' wire - should be blue - for this purpose. as described in my original post.

i have the bose system, what head unit do you recomend as this sony i just bought only has 2 phono's on it
if there's only 1 preout (2 channels), then you're best to use the speakerwire outputs for the audio signal to the Bose amp. though it may end up 'too loud' cos the signal is more powerful than what the factory headunit provides.

alternatively, a new headunit with at least 2 sets of preouts (4 channels), and preferably 4V preouts (but at least 2V).


29 Posts
Thanks Shiny for all your help
I have just installed a sony CDX-G929U in my 147 with bose system, used your guide and probably wouldnt have been able to do it without it!
thanks for all the PM's to as they were even more help.
you hit everything on the head, this is what i did.

i bought an ISO extension cable from halfords at £15 then cut the speaker wires on it and soldered on phono jacks (4 in total)
i then took an ignition live from the middle pin of the cigar lighter and wired that to the extension cable.

then i swapped the permanent live over in the extension cable, after that i disconnected the yellow mini ISO and connected the only wire in it (grey) to the head units REM

and hey presto, one working head unit!

i found it alot easier buying the extension cable as i could work on it without it being in the car, only one thing i need to do now and anyone else should bear this in mind is that the phono jacks stick out at the back of the head unit and i cant fully push the headunit home, i have just ordered some right angle phono adaptors and i hope they should do the trick, failing that im gonna cut a hole in the plastic behind the dash so it will fit snug (it only needs to go back a quater of an inch)
anyway just like to say a big thanks for you taking the time to write this thread.

AO Silver Member, 155 Lounge Winner 09
7,713 Posts
Discussion Starter #17
good to see you've succeeded. :)

you've confirmed that the 'middle pin' of the cig wiring in the 147 is the same as the GT, suiting an IGN source; i've amended my post accordingly.

right-angle RCA/phono terminals should do the trick. other brands of headunit have the terminals on 'fly leads', which provides more space. be careful about cutting into the black plastic behind the headunit - this is part of the ventilation system!


29 Posts
lol thought as much so dont wanting to be cutting it really as my head unit will probably overheat when the heating is on.

cheers again mate

AO Silver Member, 155 Lounge Winner 09
7,713 Posts
Discussion Starter #20
yes, it's the main powerwire for the cigarette lighter. there are 3 wires in the cig lighter plug:
1. backlighting wire
2. +12V powerwire (the wire in question): it is 'switched', meaning it is 0V when the ignition key is OFF, and +12V when the key is ON; hence, it suits connecting to the ACCessories wire on a headunit
3. GND

it is fine to share the wire between triggering the headunit on/off and heating your cig lighter if necessary.

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