Remove the inner door panel. Using a pin probe and a schematic, check for the presence of 12 VDC at the motor when you push the buttom. If you have voltage, the motor itself is dead. If not, check voltage at the appropriate “hot” wire at the button on the driver’s door. If you have voltage there, then you have an open in the wiring harness between the driver’s panel and the RR window. If you have none there, you’ll need to trace back from there.
The circuit itself will be parallel circuits (one for the driver’s door button and one for the passenger’s door button) both running through the driver’s door. The harness runs from the fusebox through “cutouts” in the body into the driver’s door and into the passengers door.
The harnesses usually wear where they go through the door cutouots, where they flex every time the door is opened. Finding and splicing the bad wire requires untying and “breaking into” the wiring bundle, then probing with a meter and a pin probe.
Note: of the motor is bad you may have to buy the whole “regulator assembly”. Many are not made to be repairable.
Good luck.