check around the fuse box to see if a list for relay is there. Not sure though, on my w201 the seat relays are located under the seats. I had a problem with my driver headrest not working, I would hear a click every time I pressed the switch to move it but nothing happened. I ended up teaching the click sound under the driver seat which actually turned out to be the relay (opening or closing) so it was functioning correctly, I ended up having to replace the motor to the headrest.

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1990 190E 3.0L