My 1990 560SEL's power seats and power steering wheel adjustment either all work fine or nothing works. Mechanics want a day to troubleshoot, but is there a central control module that could be dirty and just need cleaning? If so, where is it?

 

Before going to electronics I would start with checking the fuses.
It is common practice, that people put aluminum fuses into brass holders in the Mercedes.
Aluminum and copper (brass) makes electrocorossion.
Make sure you have all copper fuses.

 

I changed all my fuses to copper ones and the seats and steering wheel adjustment so far have worked fine. Many of the aluminum fuses were very corroded. Thanks for the suggestion.

Perry White.

 

have you tried resetting the power seats?