Not sure I can help much, but:
1. Have you tried spinning each motor's serrated wheel by hand with the motor and headlight assembly in place to see how much resistance there is then to it turning? (should turn quite freely, just takes forever to see any appreciable movement of the headlight housing) I haven't repaired any, but I understand there are large round pivot bearings/balls supporting the headlight housing that can corrode and cause binding, as could any misalignment of the attaching parts, etc. Do the headlight assemblies rotate freely with the motors removed?
2. Have you checked the harness voltage getting to the motor, and the resistance of the ground connections, to see if wiring connections are clean and getting the full voltage?
3. When you say the motors spin freely out of the car... have you applied 12v to them on the bench to see if they spin up, or are you simply spinning them by hand?
4. Is there a way you could borrow a known correctly-working motor to try it in your car?
Breaks in the windings, or breakdown of the insulating (shellac?) coating on the windings, can cause DC motors to lose power. I don't have a shop manual with me to see if it gives any test specs... it may require a professional motor repair shop to determine whether there are internal shorts or other problems.
