The internal combustion engine in most cars and trucks is cooled by a water and antifreeze mixture that is circulated through the engine and radiator by a water pump to enable the radiator to give off engine heat to the atmosphere. Some of that water is diverted through the heater core to transfer some of the engine heat to the passenger cabin. 

Heater core (arrowed) in the partially disassembled dashboard of a BMW E32.

 

Where is the heater core located? The heater core is a small radiator that is mounted inside a "heater box" or "hvac box",  and generally located beneath or behind the dashboard of typical passenger vehicles. On some cars and trucks, especially on older models, the hvac box is directly accessable beneath the dashboard or through the glove compartment, and gaining access to the heater core can be as simple as removing a few screws from the hvac box to expose the heater core.

However, on most modern vehicles, the hvac box is located between the dashboard and the firewall. On these vehicles, the dashboard must be moved aside (or removed from the vehicle completely) to gain access to the hvac box. It has been said that on some models the manufacturer started with a heater core hanging in the air and then built the entire vehicle around the heater core. As this implies, it often feels as though you have to completely disassemble the vehicle in order to reach the heater core.