I think that the key to your situation lies in a part of your posting - you rebuilt the wiring harness for it yourself because it was crumbling.

Your car is a 1993 so this sounds to me like your engine bay wiring harness has fallen victim to the well-known deterioration of the plastic insulation around the wiring. 1993, 1994 and 1995 Mercedes vehicles were subject to this. I have a 1993 300CE Cabriolet that had to have the engine bay wiring harness replaced last year. It was done at the same time as the head-gasket, largely because my very dependable mechanic told me that it is the best time to do it (he didn't charge me for ANY labor for the wiring harness) AND because he said he wouldn't want to do the head gasket unless I did the wiring at the same time. I sucked it up and put the money into my "baby" and had both done.

The insulation becomes so brittle that it begins to flake off of the wire so taht you start to get current crossing over from one circuit to another. Part cost for the wiring harness is around $850-900. I believe that the forum has some DIY instructions on how to do this yourself. Supposed to take a few hours if you take your time and be careful.

Best of luck.

__________________
Jay Yambrovich
Scotts Valley, CA.

1993 300 CE Cabriolet (A124) 131K miles
1997 C-280 133K miles

2000 BMW R1100RT 69K miles
1989 300 E 216K miles (sold)