Several years ago I had the exact problem on my '81 300SD. I removed the seat and alligator clamped the motor to my battery charger. For some unknown reason when I hit the right combination, it magically began moving forward and backward. I put the seat back in the car and it has worked for some ten years.

Am guessing that there was some corrosion in the wiring someplace and the seat removal and testing rubbed off the rust. Otherwise no idea.

So you might try removing the seat and connecting up the motor and trying to run it in all directions. If that does not work, then look for a busted gear.