The real prlem with the rear roll down window is not the motor or the "belt" which is actually a perforated plastic strip.
The problem is usually the Rubber blocks that are glued to the glass that engage the plastic strip, either the block separates from the glass which is easily fixed, you glue it back on
More likely the little cut fingers on one end of the blocks get brittle and snap off, when that happens you are basically screwed.
The other thing that goes wrong is they used rubber "Jack straps" to provide tension
and push the window upwards and these age and break too...
When they do the plastic strip and the rubber blocks have to do the job on their own and failure follows...
I have one of these window assemblies and eventually I'll have time to figure out how to re-engineer it so it will actually work....