Location of 1998 Chevrolet S10 4.3 engine thermostat?

The thermostat on a 1998 Chevrolet S10 with the 4.3L V6 engine is located on the engine, at the very back (rear) of the engine, on the passenger side. It's usually part of the intake manifold housing. You'll likely need to remove some components to access it properly, potentially including parts of the intake manifold or some associated hoses. A repair manual specific to your year and model S10 will show you the exact location and provide detailed instructions.