Hi Rick,
I had exactly the same problem with my 1990 300 SEL. I purchased the car about 3 months ago and when I got it I noticed the right (passenger side) headlight wiper was unplugged. When I plugged it in I continually and almost immediately blew the fuse. I did some research in the forum archives that said there was some kind of relay integrated into the right headlight motor. Anyway, I had a spare used right headlight wiper motor. I changed it out and it corrected my problem. It has been working perfectly for since then. I can’t tell you why, but I would bet if you replaced that right headlight motor 9with a good one of course) it would take care of the problem.
Hope this helps.
Outofspec