I recall working on my 68's windows. There are only 2 wires, and indeed the polarity reverses when the direction of motion is reversed. In other words, thee are only 2 wires to the motor, and the switch reverses polarity. The 72 may be different... D^2 my '64 Merc and '76 > Eldorado that the window lift motors have two wires, one for up and one > for > down. The negative ground is accomplished by a ground strap at the base > of > the motor. When positive voltage is applied to one of the wires, the > motor > is activated. When positive voltage is applied to the other wire, the > motor > runs the opposite direction. >