Just did this job myself yesterday and changed all three bulbs but don't know why I bothered, the two gauges that were fine before now have no illumination
The moral is to TEST before you put back together!
I have seen a guide here somewhere but probably easier to tell you what I found. Mine's a 2003 and has the same silver dash centre as yours, I think. It is a fairly big operation to change the bulbs but you may as well clean/inspect things along they way!
- Get radio removal tools - two C-shaped rods - or cut up a wire coathanger to suit. Use these to release the radio, being careful not to scratch the faceplate. Make sure you know the radio code (if applicable) - mine's 5115
I downloaded software to calculate that from the serial number. Remove the radio and keep it out of the way (too easy to scratch...)
- There are two small recessed-hex-head screws pointing upwards, either side of the radio aperture. Use a suitable Allen key to remove.
- There are four Philips screws below the radio aperture. I think I had to remove the radio mounting sleeve, which involved bending it to a different shape to get it out through the hole. Two of the screws secure the climate control panel, the other two secure the silver trim above (which is what you are trying to get out).
I think the two that secure the silver trim are masked by the climate control panel, in which case, remove the climate control panel to avoid straining/breaking anything...
- Pop up trim around gear selector (mine's a Selespeed). Use fingers through the gear selector leather boot to pull the trim upwards (including the mirror switch/foglight/ASR buttons etc.) It seems to snap out fairly smoothly, no huge risk of breakage, but I'd avoid prying at the edges with a screwdriver as that could easily mar the silver finish. No need to remove completely (I couldn't figure out how to get the Selespeed's tiny knob off!)
- Remove two Philips screws at back edge of black console under trim just removed.
- Open ashtray and remove two countersunk screws in upper corners. Pull the ashtray unit out and disconnect white plug on back. Now is a good time to clean dust from edges, of course, and also check for a broken hinge (mine was) - I repaired mine using a new style of glue - basically, superglue dropped onto a fine black filling powder. Sets instantly, very strong, much better than Araldite.
- Remove two more Philips screws behind ashtray assembly just removed.
- Move black console slightly rearwards to access two Philips screws under climate control panel. Pull climate control panel out. Not necessary to remove completely unless you wish to take it apart and pack/grease the control knobs to reduce their notchiness as I did
- Silver trim around gauges should now come off to reveal four screws holding the gauge cluster in place (between and below the gauges). There is one plug on the back. The bulb holders are the twist-to-remove type, easier with long-nose pliers.
You need wedge-base capless bulbs, 12V 1.2W, type 286, as shown on Gazza's excellent page Gazza82's Alfa 156 2-litre Twinspark
Again, I stress, test the bulbs before putting everything back, so that you don't repeat it all in a week's time like I have to!