Where is the thermostat located on a 1991 Chevy Caprice Classic 5.0L?

The thermostat on a 1991 Chevy Caprice Classic 5.0L is located on the engine, at the very bottom of the engine, on the passenger side of the engine block. It's usually housed within a thermostat housing, which is bolted to the engine. You'll need to look closely below the intake manifold on the passenger side. Access might require some maneuvering, possibly removing some other components to get to it properly.