The nylon gears are torn up.Replace the motor.Or if you feel adventurous,just replace the top of the motor containing the gears.
It is easier to remove the whole regulator out of the door to do it all.
CAUTION!!! the spring is under extreme pressure !!
When you take it out of the door,put the regulator in a bench vise TIGHT so not to undo the spring and not to lose an eye or an arm.This will make the removal and replacement of the motor easier.
Remove the motor from the regulator and make the necessary repairs.
Any power window from '66 all the way up to '89 RWD car will work.Even motors from C bodies.
If you think the motor is running but the gears are stripped,undo the two long skinny bolts holding the motor together.From a donor p.window motor swap the head.Make sure the head is for the same side window.(thay are interchangeable-been there done that with 3 p.w. cars for 6 years)
I hope this helps,good luck!!
Sincerely,Leaburn