side window motor is fine but will only go down

I took out the rear driver side window motor and tested it on another windows power and it works find but will only go down when i use the switch. I cleaned the switch and it still persisted and then I switched switches completely with another window and the same thing happened. Is it a wire? any help would be appreciated

 

Your regulator is more than likely dying. Going down = no resistance. Going up meets all kinds of resistance. Time to replace it. Buy two and do both fronts to save you the headache later.