I had uncontrolled movements of my driver's seat on my '89 300CE as well. If I opened the driver's door with the car off it would reconfigure itself and if I tried to stop it by moving the seat in the opposite direction it would stick and continue moving until I closed the door. It would also reconfigure itself after I closed the driver's door and turned the ignition on. It would store memory presets for a while and then loose them. On one or two occasions it moved while I was driving (luckily I was stopped at the time). I replaced the entire seat switch assembly a few weeks ago with a good used one and I have not had the unwanted seat movements since. I'm hoping it was just the switch. I also grabbed the power seat module just in case. I would start off with replacing the switch if you can get a good one inexpensively since it is quite easy to replace. If that does not cure the problem look at the power seat module and associated wiring.

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1989 300CE