Where is thermostat located on 6.0 vortex 2000 Chevy silverado 2500?

The thermostat on a 2000 Chevy Silverado 2500 with the 6.0L Vortec engine is located on the engine, at the back of the engine, on the passenger side. It's typically part of the intake manifold housing. You'll likely need to remove some parts to access it, potentially including the coolant fan shroud and possibly some other components. It is not easily accessible and will require some mechanical aptitude to replace.