Hmmmm

I would replace all your fuses. The fuse box is located under the hood and you can get a full set of fuses for the car cheap. They are unsealed and tend to get corroded with age. I have a 1982 300SD which is the diesel version of your car. When I first got my car it had all sorts of oddball electrical issues like that until I replaced all the fuses. Get the OE ones as I tried the glass fuses at first and got a lot of blow outs because they were not quite 100% to factory spec. Other than the fuses the other place to look would be at the switches themselves. You have to remove the center console panel in order to get to them. The my 1982 has a different style of switch than your later model so you might have to replace rather than take them apart and rebuild them.

My 1982 is still alive and kicking and is my everyday car at the moment. Photos at http://beostar.paunix.org/newmbzpics