Where is the thermostat on a 2001 gmc sierra 1500 5.3l

Follow the top radiator hose towrds the motor. you'll see a hose clamp then on the otherside of that is the thermostat housing. there's two bolts holding it on. remove that and any other little brackets that are in your way and it's right there under that housing. Drain a few gallons out of your cooling system, remove housing, and pop old thermostat out. Clean the block and housing with some emery cloth, like $0.40 at auto parts store. (Note, that might not be needed if it's an o ring instead of a gasket. People at store can tell you for sure.) Once it's shiney, place new thermostat it put on new gasket and reinstall. If o ring, place o ring on thermostat and reinstall. top of cooling system, start truck, and keep topping off fluid till it's blowing good heat and not overheating.

Follow the top radiator hose to the engine block and where it meets you have to take that off and the thermostat is inside. here is how.
http://www.autozone.com/addVehicleId,2561703/initialAction,repairGuide/shopping/repairGuide.htm?pageId=0900c152801cac3b

Follow the LOWER radiator hose from radiator to engine block. Where the hose clamp is that is the housing that holds the thermostat.