Rear window problems is a big topic of discussion with Bronco's. It could be that you have a bad gear in your motor, or a bad motor period. Either way, you'll probably end up tearing into the tail gate to fix it. I don't know if the later Bronco' have a relay someplace that may be bad, my '79 did not. I had a bad gear in the motor, replacement gear cost me about $10.00 at the parts store. Look for broken or chaffed wires, especially where they go from the body to the tail gate.