Where is the thermostat on a 1999 Chevy express?

Follow the upper radiator hose from the radiator to the engine. Where the hose is attached at the engine is the thermostat housing. Drain some coolant from the radiator thru the petcock, remove upper radiator hose from thermostat housing, remove thermostat housing then remove the thermostat. On the Express van you will probably need to remove the air filter assembly to gain room to work.