Where is the thermostat on a 1991 ford ranger xlt 3.0L v6?

The thermostat on a 1991 Ford Ranger 3.0L V6 is located on the engine, at the very back of the engine, on the passenger side. It's usually buried somewhat, making it a bit difficult to access. You'll likely need to remove some components (possibly hoses or other parts) to get a good look at it and replace it. A repair manual specific to your vehicle year and engine is highly recommended as the exact location and removal procedure may vary slightly.