The heater core is a main component of the cabin heating system. It is much like the radiator only smaller and for the opposite purpose. The radiator is meant to cool the engine. The heater core is a coiled pipe that sits in front of a fan. Hot coolant from the engine runs through it and the fan blows over the hot pipes and produces heat that enters the cabin. The heater core can be relatively difficult to get to if it accessible from the engine compartment. Some cores may be accessible from behind the dash. The removal of the dash is required in this case. If the core is accessible from the engine compartment, there may be limited space to work in and several large components must be removed before it is accessible. Some experience may be necessary to do repair work. At the very least, a service manual should be consulted.