I had a similar problem on my 93 300e, passenger tail light kept going out. I ended up completely disassembling the light fixture and found that the metal piece inside that the bulbs seat in was off of a pin at one point, allowing it to flex and not make contact with everything it was supposed to. If I remember right it's a keyhole type attachement, and you're able to slide it to one side to remove it. I also found the snap cover on the wiring connector was loose, allowing the contacts to float around a little bit.