Does it sound like the seat motor is working when you push the switch but not moving the seat or is it silent when that particular function is used?

If it's silent the trouble might be in the switch. Those multi function switches tend to get touchy when they get old. Tomorrow it may work fine, then something else might stop working unless you jiggle it, etc.

Mine started doing that (1991 300E) a year or so ago. The switches were kind of pricey so I took the the one switch apart and found that several of the soldered joints had cracked. I reflowed the solder and it works fine now. The drivers side has started to get funky on me now so I guess I'll have to do that one too. As I recall, they're a little tricky to get apart but it wasn't too bad.