Where is the thermostat located on a 2002 Chevy silverado 4.3 liter engine?

The thermostat on a 2002 Chevy Silverado with a 4.3L engine is located at the rear of the engine, on the passenger side. It's typically housed within the intake manifold. You'll likely need to remove some components to access it, such as the upper radiator hose and possibly some other parts depending on the specific engine configuration. It's a relatively involved job that usually requires some mechanical skill.