similar problem - solved
On my 85 500SEL I had similar symptoms. Both front switches stopped working, along with the power seats. Checked my fuses, and it was clear they were OK. The odd part about it was that the power seats would work if the engine was off and the door was open. Turns out this helped diagnose the problem.
Found out there is a bypass in the circuits to allow the seats to adjust before you get in the car. In my case there was a short under the fuse box that was causing all the problems. Not the motors, switches, or fuses at all.
Check to see if your seats operate with door open, and key Not in the ignition. That may point you to a problem under the fuse box as R Eldrige suggested earlier.
Also - Radio Shack sells an aerosol electric parts cleaner that does a great job on these old switches. Just be careful not to get it on your interior.