Where is thermostat on engine of 1992 Mercury Grand Marquis?

The thermostat on a 1992 Mercury Grand Marquis (which likely has a 4.6L or 5.0L V8 engine) is located on the engine block, at the lower end of the intake manifold. It's usually housed in a thermostat housing which is bolted to the engine.

You'll need to look closely at the area where the upper radiator hose connects to the engine. The thermostat housing is often situated near that connection point. It might be somewhat difficult to see without removing some components depending on the exact engine configuration. A repair manual specific to your vehicle's year and engine size will give the most precise location and will help you with the removal and replacement procedure.