If the door switch isn't the source of your problem, I would suspect the alternator brushes are worn down and not making good contact. The fact that your battery isn't charging properly points in that direction.

When the brushes failed in my '92 300e, the first symptom was radio static, then the windshild wipers stopped, then then the car died on the road and I had to have it towed home.

The good news is that the brushes can be easily replaced without removing the alternator. This is one of the few perks of owning a Mercedes. They come as an assembly that includes the voltage regulator for about $25 if I remember correctly. Working from under the car you can access the two phillips head attachment screws. Be sure to spray some penetrating oil on the bolts because they might be a bit corroded.