the '99 and up trucks and '00 and up new style burbs have a cable system to operate the power windows. the cable is notorious for breaking. normally the LF window dies first, but it could happen to any window as they all 4 use the same set up. if the cable is broken you should be able to hear the motor running, but the window wont move. if that is the problem the good news is that it is one of the absolute easiest motor/regulator setups to replace. the bad news is it is about a $200 part from GM. They have recently started to offer just the cables, but i have never changed just one of the cables, so i cant say how difficult it would be to replace. truckernate's method of checking the motor is a good one, but remember that the motor is a reversible dc motor, applying B+ to one side and ground to the other should make it go one way, reverseing your connections should make it go the other direction. also, being that everything on that vehicle, including the windows, is computer controlled, so before you start testing the harness side of the connector find a wiring schematic or you could cause more problems than you fix. occasionally, if the brushes are stuck, you can smack the bottom of the door with your fist and make the motor run. if that is the case the motor is junk and needs replaced. generally that only happens with older window motors that have been getting slow anyway. good luck