Some things that you stated that dont make sence to me are: you said you have 12v up and down, and the motor works when removed and plugged in on the opposing side. This would suggest a weak motor to me, but then you say if you switch the wires it rolls up. What wires are you switching exactly? At this point id say you are either dealing with a weak lift motor or one screwy wiring problem.
......And just as they're about to catch me.....Twin......Turbos......Kick.......In!