Similar problem. Removed and stripped the motor, which then seemed to work without a problem, but it still didn't work when back on the door. The problem - The wire had snapped in the "hinge" bit after 20 years of opening and closing, so I managed to make quite a long job out of a simple fix!

You may have fixed this by now, so am sorry if this is irrelevant.

Check that the connector prongs are held firmly where the loom plug goes into the motor body.
I bought a new motor(cheaper variety) and one of the pins wasn't held properly so didn't engage properly when connected - just got kind of 'pushed back'.
I opened up the motor body(think there's a plastic screw cover) and prised the flange on the base of the pin out a bit till it was held firmly.
Did the job and the motor works fine.

Good luck!