Before checking out the ground, do checkout psfreds thought on the over voltage relay etc. It sounds like an excellent guess and changing that is an easier job than trying to find a bad ground (which could be the problem but...dont know). You can check for repair CDROMs on ebay, search on your car model and look for manuals, etc. Also FastLane here may sell one from MB. Finally I think MB (the dealer) can order a printed manual for your car. On my old 1984 380sl the over voltage relay is located on the pasenger side above the fuse box behind an easy to remove panel. You might google and search here to see if someone has diy nores on where the relay is.