Where is the thermostat on a 2000 Chevy Silverado 1500 5.3 engine?

The thermostat on a 2000 Chevy Silverado 1500 with a 5.3L engine is located on the engine block, at the lower end of the engine's coolant passages. It's not easily accessible; you'll need to remove some components to reach it. You'll likely need to drain some coolant as well. This is usually a job best left to someone with mechanical experience. A repair manual specific to your year and engine will give you the most accurate location and procedure.