Here's what I would do for a car I was planning on driving for a long time and if I needed to save money:
I'd go to a self-serve junkyard and harvest a good looking window motor. I'd take it apart and examine the carbon brushes. If the brushes weren't at least 50% or better I'd harvest brushes out of a motor from one of the back doors and change them out. 9 time out of 10 it's the brushes that wear out and cause the motor to stop working. Heck, I'd probably take my own motor apart first and see if that's the problem....maybe you just need to replace the carbon brushes. Sometimes the brushes get all dusty and hang up in their tracks...won't move. A little electrical contact cleaner and dielectric grease often fixes that problem. Sometimes just a few solid hits with a hammer can free up the brushes for awhile.
Brushes are the 2 little rectangular pieces with a somewhat square cross section and a braided copper wire coming off one end. They make contact with the motor commutator.
see photo here:
http://www.gearseds.com/curriculum/i...nd_brushes.jpg