The motor has a "hall" effect counter built into it. IF the motor is installed at the point where the counter is at the min OR max amount, then the window won't synchronize & the glass will kick back as you described. The motor will have to be removed from the regulator then run the switch to change the motor's position so that tha counter can be brought back into synchronizination.
OF course the door panel must be removed to do this.
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)