For 1990-2009 cars

The heater system heats the air that enters the vehicle passenger compartment. The heater core is the main component of the heater system. The heater core is a small radiator that is located inside of the passenger compartment. Whenever the engine is running, engine coolant pumps to the heater core directly from the engine. The engine coolant transfers heat to the heater core tubes and fins. Air, that passes over the fins on the way to the passenger compartment, absorbs the heat.