The CAN bus is a serial data bus that controls accessories on new cars. That way they don't have to run power wires from every switch. On the motors there is a CAN transceiver module that receives the commands to go up or down. It is integral with the motor. So essentially you would change the motor. However, I don't think anyone sells just the motor. They do however sell just the cable/track mechanism so if you are buying aftermarket be careful what you are getting. I cannot guarantee that is your problem but the symptom is the same as I had and I think MB Doc would tell you the same thing. Maybe he will chime in on this matter.

__________________
1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra