The heater core is a smaller version of the radiator that is used to keep your toes warm when it’s cold outside. It is mounted under the dashboard. Some of the hot coolant is routed through this little radiator, by more hoses. A small electric fan is also mounted there especially for the purpose of directing the heat inside the car. The principle is exactly the same as the one used in the radiator for your engine, except that the heat is released inside the car instead of outside.