Where is the thermostat on a 1989 Chevy truck?

In a 1989 Chevy truck, the thermostat is located in the engine, inside the engine block or intake manifold. It's not something you'll see easily without removing some components. You'll need to consult a repair manual specific to your 1989 Chevy truck's engine (e.g., a 350 small-block, a 454 big-block, etc.) for the precise location and procedure for replacing it. The manual will show diagrams and steps to access it.