I recently had a similar problem with the driver's seat. I bought a new seat control since it is a very common problem, I still had problems, I bought a complete new (to me) seat and I still had problems. It turned out to be one of the female electrical connections between the seat and the car was too wide to make good contact with the male pin. There should be two electrical connections in the front under the seat. Try that first...In case that it is a bad motor and/or seat control, I may have all the parts you need. I can let go for cheap... Let me know and good luck.
P.S. Do a search, I posted some pictures recently...

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J.H.
'86 300E