A few things.

Have you checked for power at the motor with an incandescent light? ( digital meters / LED test lights will pickup stray voltage and give the impression that you have power but when a load is applied , it falls off. )

Tap on the motor while having the switch in the forward position. If you have a stuck brush this might be enough to make it move.

I don't know how your car is set up but sometimes the motor can be unbolted and armature turned to move the seat.