What I like to do is, in a dark environment (at night or in a closed garage) turn on the ignition but don't start the engine (so that the power window system is energized) and the overhead courtesy light. Press the power window button of the non-operating power windows just for a second...to see if the courtesy light dims ever so slightly. Do this in both the up and down sides of the button. If the courtesy light dims when you press the button it is a sign that the electrical circuit is complete to the offending window but that the window motor itself is stuck (probably needing to be removed, disassembled and cleaned). If the courtesy light does not dim when holding down the offending power window button, then chances are that the window motor is not getting any electricity and that the fault probably lies in the switch itself or the wiring (possibly at the door fles point).
The above method has served me well in keeping my window systems properly maintained.
It is not uncommon for rear window motors on old cars to suffer a little rust. Most of the time you can disassemble and clean (and free-up) the most with great success! Plus...you get a lot of experience on removing and installing Mercedes door panels (which is another discussion altogether).