I just went through this withe my '92 400e. There is a little gearbox that is driven by a flex drive shaft from the motor. Each motor drives the 2 matching gearboxes, one from either end of the motor. One motor drives both rear seat elevators, for example. The square end of the center shaft barely engages the square hole in the mating part (worm gear) in the gearbox. They can strip out at that junction, leaving one seat elevator working and one not. I just pulled the shaft out a bit and reassembled, I assume it will fail again at some point. Still haven't chased down a new part number, if it's even available.

Run the seat all the way forward, and raise the rear as high as it will go, drop the front as low as it will go. Remove the plastic covers and the gearbox is right there. Take the side cover off and the problem will be apparent.