With the front windows there is luck because at the lower bottom is a junction for the window motor wires. A pic in post 4.
http://www.peachparts.com/shopforum/vintage-mercedes-forum/407989-w123-electric-window-issue.html
On one door I had the screws get loose and the window motor stopped working.
Unlike your starter motor the grounding the window motor is no part of the circuit. It all goes through those 2 wires. Applying a wire to a positive and a negative on a better gets the motor to go one way and swapping the wires around and the window goes the other way. To do that on the rear you have to locate which of the wires in the switch socket go to the rear motor.
In post 3 is a picture of a window fuse that looked good but when I pulled the fuse the metal nose of the fuse was gone. So, they can look good from the top but are making bad contact on their noses and brackets.
http://www.peachparts.com/shopforum/diesel-discussion/261435-w123-window-motor-removal.html
Back to the junction block on the front door. If your remove the motor wires and you have a digital voltmeter you can connect one lead to one terminal and one lead to the other terminal and put it on whatever setting measures 12 volts. From there you can have someone else work the switch and see if the switch works in both directions. On the meter you should see the polarity change.
I don't know how to do that on the rear. I likely would need a wiring diagram or tap into the motor wires to do that.
I had the front passenger side motor totally frozen up. The lubricant had solidified.
Next is the window actuators. The rear ones bend and all have plastic sliders.