The thermostat on a 1999 Ford F-150 4.2L V6 is located at the
rear of the engine, on the passenger side. It's usually housed within the intake manifold. You'll likely need to remove some components to access it, such as parts of the upper intake manifold. It's not a simple, easily visible location like on some engines.
Accessing it requires mechanical aptitude and the right tools. A repair manual specific to your year and engine is highly recommended before attempting this repair. The manual will provide detailed instructions, diagrams, and torque specifications to ensure you do the job correctly and avoid damaging other parts.
