Ford Excursion :: One Power Window Will Go Up But Not Down

I bought a nice 2000 Excursion with the 6.8 V-10 a few weeks ago, and got it really cheap because it had a few things the previous owner just couldn't figure out.

One issue the PO couldn't resolve is the passenger side rear window--he recently spent money having all new lock actuators and window motors put in because a couple of them had quit working, but the right rear still won't go down and he didn't want to spend any more $$ figuring out why. All other windows function normally, and I believe the right rear would go UP if I could ever get it down because the problem is only with the lowering voltage, not the raising voltage.

First I checked at the right rear motor connection and toggled the right rear door switch--I had 12v to raise, but nothing to lower. I swapped switches with the left rear door, a known good motor and switch...same result. I checked voltage at the INPUT to the RR switch....same result--I get 12v to raise, nothing to lower. So I moved to the next logical place, the master control (drivers door switch). On the output side of the switch, voltage toggles between +12v and -12v as it should to raise and lower. Everything is good from the time it leaves the driver's door switch, so apparently I'm losing my lowering voltage somewhere between the output of the driver's window switch and the input of the rear passenger window switch. I checked for chafing/loose/corroded wiring all the way from the B-pillar to the right rear door switch, a common cause of this symptom, and everything looks fine for that stretch of wiring.