I just rewired the 300 TE for a sound system and when I put everything back together I had a problem. I didn’t think I screwed anything up, but at that point it seemed logical that I had.

The following switches did not illuminate when the lights were on:
rear dome lamp switch
fader
heated rear window switch
window switches in console
rear wiper switch
rear wiper washer switch

All the lights in the cluster worked and dimmed fine.

I couldn’t figure out what was going on so I took it to Ted at Black Forest in Charleston, SC for him to diagnose. He tested a few wires, the grey/blue ones, at the junctions, the ones that attach to the front of the evap case and the ones that go to the junction on the left side of the center console. After reviewing several wiring diagrams he said we need to take out N40 which is the Instrument Illumination Control Unit. He thought that 58D on the unit was not putting out power. It’s located behind the instrument cluster which had to be removed. To get N40 out you have to remove the bracket that the Control Unit and the Hazard Flasher Unit are attached too. Kind of a pain since the fasteners that hold it in are hard to get to, one is a 10mm nut that attaches to the firewall, it goes over the same stud that holds the evaporator case to the firewall.

He took the Control Unit apart and inspected it under one of those lighted table mounted magnifiers. He found a small crack in the solder around the base of , I guess it was where a transistor or something came thru. He showed it to me and I had to look at it for several minutes before I could even see what he was talking about. It was a very tiny crack.

He soldered it and after it was put back in all the switches illuminate as they should.

I can fix lots of stuff, but when it comes to diagnosis, and for me especially electrical, you have to be very experienced and pretty darn sharp. Wish I was more of both.

So it you are in Charleston, SC and need any work done on your MB I highly recommend Ted at Black Forest. http://www.blackforestimportservice.com/