Where is the thermostat located on a 1990 Buick Century 3300 V6?

The thermostat on a 1990 Buick Century 3.3L V6 is located on the engine, at the bottom of the engine, at the back of the engine, on the passenger side. It's part of the engine's water pump housing. You'll likely need to remove some parts to access it, such as a fan shroud and possibly some other components. A repair manual specific to your year and model will give you the most accurate and detailed instructions for accessing it.