The thermostat on a 1991 Ford Tempo is located
on the engine, under the thermostat housing. To be more precise, you'll need to look at the top of the engine, near the water pump and the intake manifold. The thermostat housing is usually a metal casting with a few bolts holding it to the engine. The thermostat itself is inside this housing.
Because there are variations depending on the specific engine (2.3L or 1.9L), a detailed diagram in a repair manual specific to your 1991 Ford Tempo would be helpful for precise location. You can find these online or at an auto parts store.
