Coolant Temperature Sensor
WARNING: This page is about a different car, the 2002 Mercury Villager. However, it is still accessible from the selected car via links, so may be relevant.
Coolant temperature sensor is a thermistor-type sensor, located on heater inlet hose between engine and heater core. Sensor tells EATC module when engine coolant is warm enough to provide warm discharge air to passenger compartment.