Re diagnosis, measure w/ DMM between the 2 motor wires. There is a convenient terminal block inside the door which makes access easy, plus your door card is already off. If you measure <8 V while trying to actuate the "UP" motion, search for the voltage drop. If your front right window shows the same issue, the drop is likely in the fuse contacts.

If using silver fuses instead of gold, this is common. Ditto for sun-roof and other loads. Remove the fuse, sand the ends of both fuse and holder, coat w/ silicone grease and re-install. That fixed many of my similar problems. Someday, I'll have all gold fuses.

As mentioned, could also be a broken wire in the door feed-thru. The window switches can be taken apart fairly simply and sanded of corrosion, but don't fool w/ that until you verify a problem with your DMM. In you don't own a DMM, a trip to HF w/ their coupon will yield a freebie.

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