97 Taurus rear power window only works in cold weather?!?

[GoLow]

I've got a '97 GL that, from the start, the left rear window only works when the outside temperature drops to around 45 F. The rest of the year it won't work at all. When I press either the front master or the left rear control, I can hear the motor and the interior lights dim slightly. 3.0 OHV, nothing fancy......Ideas?