Probably not a fuse because the L rear window shares a fuse with the R front window, so if that works it the fuse is probably OK. But, I have found fuse problems from the factory silver fuses. I measured 12 V on the switch supply lead, but it dropped <1 V when the switch was actuated, and a high-resistance fuse contact was the culprit. Replacing with a gold fuse fixed those diagonal windows. It might be such that the current load of the R front is slightly less so it works while the L rear doesn't.

Anyway, you can do most testing from the console switches with a multimeter and a schematic. You should measure <100 ohm across the motor leads if wiring and motor is good. The rears have funky wiring because of the "rear lock-out" button in the front. But first get them working from the console switches. I found broken wires in the jamb feed-thru of my 1985 driver's door, which is a common problem. I am lucky my driver's window hasn't broken again since I must lower it 3 times on the way to work going thru security checks.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans