Where is thermostat located on 1991 Mercury Cougar 3.8L engine?

The thermostat on a 1991 Mercury Cougar with a 3.8L engine is located on the engine block, at the lower part of the intake manifold. It's usually on the passenger side (right side when sitting in the driver's seat), but you'll need to visually inspect the engine to be sure. It will be a housing with a hose going in and out, often with a single bolt holding it in place. It's often a relatively easy part to locate once you're looking for it.