If you can hear the motor running, then it will probably be one of the following.
1). Stripped gear on motor output
2). Stripped gears in motor gearbox
3). Stripped teeth on the window regulator
You could be lucky, and something might have come loose with the regulator, but the only way to find out is to remove the door trim and have a look.
I doubt the dealer caused this by action or lack of action, this was probably just waiting to happen.
Either way, it's gonna cost to fix. I just replaced a motor, and it cost me $200 for the motor, and I fitted it myself !
1989 W124 300E
1991 R129 500SL