If your car was designed by the same people that did my 1987 300D, there is a module in the relay box that contains nothing but a single diode. On the 300D it is the furthest back, hardest to get to module. Also, on the 300D you get to the relay box by taking out a few screws - it is in the same big box as the fuses, but it has an additional cover.
The "diode" module provides a signal that causes power to be provided to the power seats with the doors closed and the ignition on. If someone has pressed down too hard on this plastic module, it can break the diode.
I know because I did just that, and got the symptoms you describe. It is possible if you are good with mechanical skills that you can take apart this module and replace the diode. It is only a two pin module. I had similar diodes on hand and soldered one in - a free fix.
If you are familiar at all with old crystal-controlled radio transmitters, the module looks a lot like an FT-243 crystal holder as shown below: