The heater core is generally located inside the dash of your vehicle. This small unit is in fact a small radiator that draws hot coolant from the cylinder head. The heater core with the support of a blower sends heated air it generates into your vehicle. This system runs regardless of thermostat. If you are ever over heating, turn on your fan and crank the heat to send some of that hot air out of your engine block! Heater cores while generally inexpensive can be time consuming to replace.