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Discussion Starter #1
The rear 12v socket in my 159 SW has stopped working. Its not the fuse, I must have damaged the wiring somehow when investigating polarity in previous jobs last year.

My question is about wiring: What route does the wiring take from the battery to the cabin in the 159? I would like to add an extra 12v socket for the rear seats of the car and I would like to be always on (not cut out with ignition off) as I have a solar panel for trickle charging the battery while I'm away from the car for long periods (airport car park).

Ideally, I would like to run a new cable from the battery terminals to the 12v socket. I plan to include an in-line fuse at 15a or 20a, so I can also run a tyre compressor from the same socket.

I don't want to mess up the firewall between the engine and the cabin and I don't want to run a cable that passes outside the vehicle (out near the windscreen, then in the passenger door hinge!), so is it possible to add a new cable to the existing cable loom - where in the engine bay does the wiring pass into the cabin? Could I connect from the fuse box perhaps - is there a spare always on connector in there?
 

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For the towbar electricity modules I tapped of 12V constant from the rear fuse box main cable.
This rear fuse box main cable is fused separately at the battery in the front.

Running an additional cable from this rear fuse box to the rear seats is easy and close by.
 

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Rear fuse box on SW to be found in rear right cubby hole
 

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Discussion Starter #5
I have 2 doors inside the rear right cubby hole. The highest one gives access to change bulbs and the smaller, lower one (about the size of a cigarette packet) has nothing inside, just a couple of wires going past! Do I need to remove all the paneling and the carpet up that side of the boot space? Not ideal for a fuse box!
 

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Maybe it is in the left?
If you remove the inner cover it should be in sight.
 

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Discussion Starter #7
The left cupboard only has a single access hole to change bulbs. The right one has 2 access doors. With my camera phone on video with light,I can stick it in the hole and there looks to be a black box with several wires connected. It looks as though the wires are all connected at one side only. I would estimate it to be about 1.5 x the size of a cigarette packet. It is bolted on to the metal behind the lower half of the cupboard. The access hole is in the top half and I can just see the top bolt about 10cm below the opening. Does this sound like the right place and size for a fuse box? There is no way I can access it without taking all the paneling and carpet out, or cutting through the cubby hole. It is held on with just 1 bolt by any chance? I could probably get in with a spanner and remove the 1 bolt I can see...
 

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Indeed the fuse box is located behind the cover in the right cupboard/storage panel.
It’s also listed in the manual of the car.

If you want to run a wire from the live 12V line of that fuse box you need to preferable remove that right side panel.
Not too difficult:

1. Remove black plastic boot panel which runs from left to right (2 outer screws, 2 inner screws and 2 luggage strap torx bolts).
2. Remove right side plastic panel which runs up the boot pillar, hold with 2 screws.
3. Remove right side luggage torx bolt.
4. remove right side 2 plastic clips of the panel next to the spare wheel cover.
5. Remove spare wheel cover by removing at the hinge the plastic clip at each end.
6. Fold right rear seat downwards.
7. Push right side panel out spring clips on the top side.
8. Disconnect right side boot lamp wire.
9. Pull panel to the middle of the car and push it towards the rear seat since the back of the right side panel is mounted around the rear seat lock hook.

I did this now like 5 times (earth connection check, towbar installation, reverse camera installation) but still it’s all from memory so don’t blame me if I missed something.

Then you have full access to the +12V wire going into the fuse box.

Before you start soldering, disconnect the battery.

I removed the coating of that +12V wire on 1 side, and spliced/soldered an additional wire to it so it.
Make sure you don’t damage the strands of the wire and that you properly isolate it.

The rear fuse box is used for several systems which my car does not have, such as the Bose amplifier which has a fuse F61 which is rated at 15A. Since the harness is all the same, if you have all options or not, the main fuse cable is rated for this Bose amplifier and all other user.

Therefore I used this +12V wire to power the electronic modules/lights of the tow bar which is probably max 2A during usage. So there is no risk on overloading the car’s original harness.
Again, it’s at your own risk.

Hope the explanation is clear enough.
 

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Discussion Starter #9
Brilliant guide thank you. I'll try that this weekend if I get a chance. Do you know if the rear cigarette lighter passes through that fuse box?
 

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This seems to be F44 marked as cigarette lighter on console, which is located at front fuse box driver side.
So front and back cigarette lighter share the same fuse.
 

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Discussion Starter #11
This seems to be F44 marked as cigarette lighter on console, which is located at front fuse box driver side.
So front and back cigarette lighter share the same fuse.
Ah that's right I wasn't thinking. I knew that.

Actually the rear cigarette socket and the rear wiper share a 15a fuse and the front 12v socket (next to handbrake) is on its own 10a fuse. The tyre compressor draws more than 10a and blows the front socket fuse, but it doesn't reach the front wheels from the rear socket. This is why I was investigating an extension and checking polarity last year when I blew something and broke my rear 12v socket. Hence this thread!
 

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Discussion Starter #12 (Edited)
I've got all the panels off, but I'm not sure that is a fuse box. Can someone confirm from the attached photos that this is the correct thing please before I open it up? It has only fine wires attached and I would expect a series of red/black wires if it were a simple electric fuse box.

Also, why would it not be positioned directly behind the access hole, rather than having an empty access hole and this 8 inches below!

Also, figure 62 in the manual is the rear fuse box. I have none of the heated or electric seat options on this car - should I still see a fuse box. Also the manual says its on the left side, but the access cover/hole is definitely on the right side in mine.

2 photos attached.
 

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Discussion Starter #13
I can see a part number 50504760 on this box - google suggests this is the rear parking sensors. I don't think this is a fuse box. Any one got any other ideas?
 

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It's not the fuse box.
My car is left hand drive and it is located on the rear right side as where you removed the panels.
Indeed figure 62 in the manual shows a picture of the fuse box which we are talking about.

If your manual says on the left side I would check that side, maybe the access cover/hole is there only for the fuel cap red rope (as shown on your first picture) which is on the right side as on mine car.
 

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Discussion Starter #15
Nope nothing on the left side either. I really don't think my car has this magic fuse box! What wiring should feed the fuse box? Can I look in the battery fuse box and see if the cabling is there? What an I looking for?
I thought this would be straight forward - I just want an always on 12v socket in the boot to replace the failed one that is there. So much for a simple job!!
 

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Bizarre taht you have no rear fuse box in right or left rear cubbyholes! That bundle of boxes is the parking sensor module.
 

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Discussion Starter #17
I can only assume the fuse box was not fitted because the car does not have the BOSE system or heated seats - the only things attached to that fuse box. I just wonder if the wiring is in place from the battery to the fuse box location - anyone know what I should look for at the battery to check this?
 

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Discussion Starter #18
One final question: I have a few metres of household wiring left over. Its standard 3core 1.0mm square. Obviously I wont need the earth write, but is that suitable for using in the car for a 15A cigarette lighter wiring? Or do I need specific automotive cabling? If I need specific cabling, what's the spec? Thank you all for your help with all of this thread.
 

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I can only assume the fuse box was not fitted because the car does not have the BOSE system or heated seats - the only things attached to that fuse box.
Not for that rerson - my Lusso has the fusebox and it is used. And I have neither of those options.
 

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One final question: I have a few metres of household wiring left over. Its standard 3core 1.0mm square. Obviously I wont need the earth write, but is that suitable for using in the car for a 15A cigarette lighter wiring? Or do I need specific automotive cabling? If I need specific cabling, what's the spec? Thank you all for your help with all of this thread.
Try looking at this: Wire Gauges - Current Ratings
I think your household wiring is just fractionally too small, but if you're not going to be running 15A to your cigarette lighter you are probably good. Your household wiring likely has the current rating printed on the side or the AWG in which case you can look it up in the table.

You may find a non automotive source of cabling (modelling) could be good if you want to put lots of current to the back of the car in a more flexible wire (multicore), or you're running new leads. Drones usually need lots of low voltage current.... I have some fat 36A multicore, but can't remember where it comes from!
 
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