ONLY an expert can repair these correctly. I have done many...I take my time and inspect every part. A toothpick could kill these things.
Anyway, the motor has a clutch in it. If the clutch is too loose, the motor will not grab the cable. You should open the old motor and see what tensions the cable onto the toothed gear. Then you will see if you made a mistake in intallation.
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS