Window motor 1991 XJ

My rear passenger window motor has an issue where it will roll down half way, slowly, then stop. When you activate the switch, either from the driver's door or the rear door, you can hear a click in the motor, but it doesn't budge. At that point, it won't go up or down. I pulled the panel off of the door and inspected the mechanics, it all seemed OK. I pulled the connection from the harness to the motor, and cleaned it up, no difference. I checked the voltage output on the harness, and it looked fine, up and down. I lubed the window channels with silicone lubricant, still doesn't budge. Finally I used another 12v source, my battery charger, and hooked it up to the male contacts on the motor. Bingo. The motor goes up and down with no hesitation. I'm thinking there's an issue with the amount of current that's being passed to the motor. Could this be an issue with corrosion in the window switches?