pull panel and look at it in action. its most likely the motor is shot. you may be able to remove motor and then lift and lower by hand if its smooth and not binding just replace motor, if its rough or binds,sticks and is hard to operate ya need the assembly ..
and here is the whole deal from amazon $54.00 shipped
OEM #15135971, Partslink #GM1551114
http://www.amazon.com/POWER-WINDOW-R...r+window+motor