Where is the thermostat located on a 2004 Chevy Express 6.0?

The thermostat on a 2004 Chevy Express 1500/2500 with the 6.0L engine is located on the engine, at the back of the engine block, near the intake manifold. It's not easily accessible and requires removing various components to reach it. You'll likely need to consult a repair manual with diagrams specific to the 6.0L engine in your vehicle for detailed instructions on its location and removal.