I've got the exact same issue and symptoms on my 1997 C230 with ~95k miles The service appointment is scheduled for next week. I would be forever grateful if someone figured out that the problem is some little thing like a blown relay. For no particular reason I suspect that's not the case, but...
A few weeks ago, I found the relay area in question behind the main fuse box but was confused about which one it was. As far as I could tell, the "fuse map card" did not correspond to what was actually in there. I think I finally figured it out by process of elimination. I went to the dealer's parts department and asked for a new one. The one I took in was green (getting a little hazy now) and they sent me home with a light blue (or vise versa) one with the same part number on it. The little prongs on the blue one did not match the green one. I plugged it in just to see what would happen. As it did not solve the problem, I took it back.
I'm not sure why, but this really bugs me. I should have taken it for service weeks ago, but I've been stubborn and wanted to figure it out myself. It's really not a cost issue. It's having to admit that something broke just after the 90k service and I have to go back before I had planned to.
If you figure out what's wrong and let me know, I'd really appreciate it.
1988 300 SL (5 Speed)
1994 E320 Wagon