It could be the power wire from the master switch to the RR switch is partly frayed or now seperated in the driver's door jam conduit, as jrford said. There are also two TSBs on this issue.
TSB 06-21-14 says to tap on the lower area of the interior door trim with a rubber mallet to 'unlock the motor'. Then run it up/down a few times, then install a foam rope into the upper weatherstriping to 'change the rate of glass deceleration'. It would also probably help to clean the leading edge of the glass real well and put some silicone cleaner on the rubber weather seal.
TSB 07-16-14 supersedes 06-21-14 and says to replace the window motor or the whole window regulator if it's damaged. I don't think you can get just the motor for the front windows, so my guess is that Ford got tired of replacing the whole regulator on the sticking rears and made a better motor available as a seperate part. Part number 6L2Z-7823394-AA, Regulator Motor Asy.
Let us know how you make out. My RR is stuck right now, but I don't have kids and rarely use the back seat in mine. So I've been putting it off. I did try rapping on the door panel but that didn't work for me. Good luck.
