Have you tried the relay mod? Sometimes it's not just the motors, but the ancient door switches. With good voltage, even the cheap motors usually last a long time.
If you're sure there is good voltage to the motor, you can open it up and bypass the safety. It's a PTC or some kind of thermal breaker. So if someone's neck is in the window or something it will stop (I always figured I would stop pushing the button if my arm was rolled up in the window, but maybe that's just me.) Sometimes the resistance gets high as they age and sometimes they just fail open, so you think the motor is dead. Soldering across that thing will make the motor faster than ever.