Where is the thermostat located on a 1999 Chevy Blazer?

The thermostat on a 1999 Chevy Blazer is located under the intake manifold, on the engine block. It's not easily accessible and requires removing components to reach it. You'll need to consult a repair manual specific to your Blazer's engine (4.3L V6 or 5.7L V8 are the most common) for precise instructions and diagrams. This is generally a job best left to someone with mechanical experience.