FWIW, after driving our Pantera regularly in the rain, I found the headlights would move very slowly either way. I watched the action with the front hood open and heard a sizzling noise when my wife hit the switch.
The stock motor is meant to operate ungrounded to the car. The up/down motion comes from the up/down microswitches, depending on which one grounds. The motor leads are extremely short and go to a peculiar bakelite insulating grommet/ vibration support bolted to the back of the motor. After 30 years, the grommet/support cracked and held a tiny bit of water.
The water boiled from high electrical current when the switch was depressed. It also partly shorted- not enough to blow a fuse but resulted in very slow headlight movement. I threw the broken grommet away and used insulated male/female push-connectors on the wires, with a tie-wrap around the motor & wires for simpler vibration relief.
No more problems although the plastic inner gear in the headlight mechanism may crack just like in window motors. Since the headlight and window mechanisms are identical, you need a third brass gear in there, too.