My Dad had a shop diagnose a bad motor on the 85 300SD even though it would work every once in a while. I just pulled the switch and saw a wire was just a little short of being fully inserted so it would contact until you hit a bump
and the either move in or out. Pushed it in and used a piece of quality electrical
tape to secure it until I have the time to take the switch apart and spread the contacts so they do not come out that way again.
Alan
