The passenger window motor in my 124 recently stopped in a similar manner.
When I applied power it would click just a little, but the motor would not turn.

A new motor fixed it right up!

Tip:
There is a round plastic bumper mounted under one of the motor bolts that will fall out when you remove the motor. I spent a lot of time trying to figure out what it was and were it went.

It is a stop-limit for the regulator. If you look at the ends of the big regulator gear there is a curved "hook" on each end of the arch.
It limits against this when fully open as well as from the other side when fully closed.

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OK, here's the easiest way to see, swap the switches between the driver and passenger side and see if the drivers side power window doesn't work much better. Get it? The drivers side power window switch is used alot more than the passenger side switch yet they are the same swich, just swap em and see. It at least eliminates the motor issue....cliff

The window just does not work...this is the rear left side. I already swapped switches to no avail. The motors are around 120 each so I want to see if I can tell for sure it's the motor before I spend anything.

Maybe I'll to to the junkyard with a battery and try to find one that works.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

For the rear windows the entire regulator/motor assembly can be had for $140ish.....don't pay $120 for just a motor!

The passenger rear on my W124 is dead, but since I really have no need to have all 4 windows open, I'm not going to fix it. The other 3 work fine. So long as the front windows work, that's all I care about.

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Well the reason I need them working is so I can get the windows tinted. Also where can I find both for 120?

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

so, it looks to me as though you have ripped your whole interior apart looking for this issue? hmmmmm....well...you maybe correct ...best of luck...cliff