I have had all my motors of all my MBs (wifs, sons and I) nonoperational at one time or another. After checking everything else and narrowing it down to the motor I found a neat trick to get the motor working again. (no doubt from this forum)

Most window motors were "frozen" due to rust issue in the shaft area.

Fill the motor body with ATF (yes it will leak all over you). use vice grips to move the shaft around. Then using a battey, jumper the motor to run in one direction.
Then reverse the battry leads to the window motor to run the motor in the other direction.
Only do this for a few seconds in either direction. Without any load the motor will spin like a tasmanian devil + burn out if run as such too long.

The ATF will lubricate the shaft/motor. All things being OK -your window motor is ready to re install and operate