Start with "less $ invested approach." The easiest way to diagnose the switch is to swap the one from the working window and plug them in sockets of the non-operational window. 0$ invested. (*You will need to pull the cover from the center console off - which can be PITA if you have armrest.)If they work, than you know it is the switch that is faulty. If the tested window still doesnt go down, than its time for more R&R.

But you say that you can hear them work...that could be stripped gear or (in case of the rear window) cable that has snapped.

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87 300D sedan rarity with the #14 death stamp SOLD