One of the most frustrating parts of working on these w126s is the way in which Mercedes decided to light all of their console switches and instrument cluster. Being high tech, they went with a fiber optic system that delivers light via an external socket. Let me get more into detail here.
The two areas that people most often come in contact with these fiber optics is the instrument cluster and the center console. First lets tackle the instrument cluster. The cluster has long been a point of contention with Mercedes owners as these cars grow older, since the lighting becomes worse and worse until the driver (dangerously) cannot read the gauges at night. Problem number one is the rheostat. The springs that the company used in the rheostat fail pretty commonly. This results in NO lights whatsoever. I recommend jumping the rheostat, which consists of simply putting a jumper wire between the two solder joints on the board. If you want to do any LED modification or anything you'd have too do this anyways to help regulate the current.
The bigger problem with the instrument cluster is the delivery of light to the gauge faces themselves. The bulbs are located in the back of the instrument cluster next to the blinker bulbs. Easy to replace for sure, but if you look closely the light has to travel through crystals over the top of the plastic housing and then refract back down on the instruments. In the bulb sockets themselves, about 90% of the clusters that I have seen have black charring (presumably from long time burning of bulbs, perhaps incorrect bulbs?) on the face of the fiber optic prisms. I have also seen the prisms melted from having too hot of a bulb in there--obviously this doesn't help either. In addition, all sorts of dust can get caught in there and is very difficult to clean. My solution was to completely bypass the fiber optics in the instrument cluster and go with a homemade LED setup. I mounted the LEDs where the prisms came out to light the gauges, and soldered them back to the bulb sockets. The LEDs were actually far too bright so I had to shield some of the light with electrical tape...even that is still very bright, but that is my preference for dash lighting.
On to the center console. All of the switches on the top of the console except for hazard lights --rear reading lights, defrost, sunroof, and antenna (may be more depending on options) are lit by means of fiber optic cables in back of the dash. There is one light strand, much like a christmas tree wire, powering all the climate control and switch lights. I believe these are wired in series so if one bulb goes out, the following will also go out. There are 4 bulbs located in the climate control unit (I will get to that in a second) and one bulb reaches back into the firewall to a cluster commonly called either and octopus or a spider. The fiber optic wires coming off of it go to each of the top switches, and one goes down into the shifter console to light the PRNDSL selection. Conveniently, my octopus was completely torn apart by the previous owner who pretty much scrapped the whole interior. After many hours of toiling I recommend just forgetting about it, you won't easily get those fiber optic cables back into the octopus--it is so far back against the firewall, it gives almost no room to work, especially for a precise placement of these wires.
Back to the light strand. The first two bulbs on the strand light the left side of the climate control--the wheel and left couple buttons. Then, the strand reaches back to the firewall where ONE bulb (stock bulbs are pretty dim) lights all of the switches and center console, then the next two bulbs return to the climate control to light the right flap buttons and fan speed/auto buttons. Since I gave up on the octopus, none of my switches light up except for the hazard lights, and I replaced my climate control bulbs with LEDs to compliment the blue feel of the car. Any questions please leave in comments section. Thanks!
