"I know the rear driver's side switch itself is OK because I swapped it with the rear passenger side switch. When I swapped it, the rear passenger side window still worked fine. "

Did you test the passenger side switch @ the drivers side connection?
I would put the rear passenger side switch @ the drivers side and see if that works. I am willing to bet it wont.

To me it sounds like the rear passenger switch is bad, or gummed up.
I just cleaned the 3 switches in my car, and the windows work much better. They were pretty gummed up. Have you taken switches apart before? They arent hard, but interesting construction.

Switch disassembly:

Black toggle switch can be removed by slightly prying on one side with small screwdriver to pop out the circular notch, and then on the other side to remove it. There will be 2 ball bearings, and 2 V looking pieces of metal. These are what transfer the power. take these out, clean all the contacts on the V piece and the small connections on the inside. I like to apply some conductive grease to the contacts. The V pieces go back in, with the ball bearings in the very middle. THe black toggle switch then comes straight down as the 2 cylindrilical pieces keep the ball bearings located correctly.

Then the outside connections get cleaned, I found that vinyl bristles cleaned it best with leaving the coating on there. Then I applied some more conductive grease and put the switches back in. Much of the time cleaning the switches helps clear up the problems.

hope this helps, it might be a little confusing. But if you disassemble the switch and read this, it make things much clearer.

Now I am stuck. I'm guessing a loose connection somewhere upstream from the rear driver's switch. Any suggestions? Thanks.

hope this helps
take care

Luke
82 242Ti 168k miles
84.5 245Ti 300k miles retired due to condition... :( now parts car :)

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