I would access the motor connections, disconnect them and see if it still happens. That would eliminate whether body wiring or motor itself.

 

Yup, prettty simple. Either the motor is bad, or the mechanism is locked up, causing a mad rush of current in the motor - but I'd still bet it's the motor.
Every car I've taken the trim out of has shown a LITTLE evidence of water intrusion. If water gets into the motor, you will need a new motor.

I can't say I've seen water damage a motor, just saying that the possibility is there.