motors on these windows do fail but it is not happening that often... what happens, is a plastic clip desintegrates over time and fails, and the glass gets stuck inside the door or falls into the door, and then, when you try to move the glass, the cable gets tangled up, and puts too much stress onto the motor, and then the motor fails...

Usually, the front left and rear right motors are the same, and front right and rear left are the same too...

Before running the motor to see if it works or not, pull the panel (there are many write-ups about it), and see if the plastic clip (about $2 from a dealer) is there... Even if it is there, it might be time to have them replaced, might even consider replacing the ones that have not failed yet...

If the cable has tangled up beyond repair, try to get a regulator off ebay, they come without motor - test your own motors (without the glas load) to see if the work, and just reuse the motors...