I have an 85 500SEL (USA) with 112k and experienced very few (but common) problems.
Recently the power seats and power windows all stopped working. When I have the car off, and door open, the seats work fully. Of course the windows do not operate in either case. I did my searches, and found many discussions about switches, motors, and the diodes.
Since they all went out at once, I doubt it is a switch.
Ditto with motor(s).
Not sure about diodes, relays?
Is there a common denominator to the windows and seats, that gets bypassed with the engine is off?
I have recently relocated - so my independent MB guru is no longer available. There is a MB dealership in town, but we all know what that means