Where is the thermostat in a 2000 S10 Chevrolet?

The thermostat in a 2000 Chevrolet S10 is located under the intake manifold, at the back of the engine. You'll need to access it from the top, likely requiring removal of some components like the air intake system and possibly other parts depending on the engine (4.3L V6 or 2.2L I4). A repair manual specific to your S10's engine will give you the precise details and steps.