Sounds like the contacts inside the motor. They either need cleaning or replacement.
Both can be completed by removing the headlight assembly and removing then cleaning the motor/gearbox interior. There are multiple videos how to remove the assembly and how to rebuild the gearbox.(youtube)
this like shows how to:
http://www.corvettemagazine.com/tech-articles/1984-1987-corvette-headlight-motor-gear-replacement/
while you have the gearbox open clean the old grease out and verify the motor armature shifts back and forth along its shaft to work the reversing limit switches. I replaced the gears and cleaned out the motor, then reassembled all works fine now.