For 1990-2009 cars

The heater system heats air entering the vehicle passenger compartment. The main component of the heater system is the heater core. The heater core is a small radiator located inside the passenger compartment. Engine coolant pumps into the heater core directly from the engine whenever the engine is running. No water valve is used. The engine coolant transfers heat to the heater core tubes and the fins. The heat is then absorbed by the air passing over the fins.