Had a similar problem with my 780's sunroof motor. Somehow, it closed perfectly, but had problems opening: it opened 1 time on 20 tries. With a screwdriver I could start him off with ¼ twist and then the motor would start.

Basically, the motor had a blind spot where internal connections probably werent able to work.

My advice: dont spend money on trying to repair the thing. Go in a sccrapyard and pull out any MY sunroof engine, from 1983 to 1992, and plug it in your car. Easier, faster and less expensive of a solution.

I was fortunate enough: my dad has a 1983 T-diesel 764 and a 1985 765 T-diesel in his backyard. The 1983 still had a fonctionnal sunroof motor, so it was just the trouble of getting mine out and the new one in. 1 hour tops.

Beware: when putting the new motor, you have to plug and install it all before you actually test the motor. If you just plug the wires to see if it works without actually linking to the roof itself, you will bust the motor.

Hope it helps. Cheers.