Heater Core
WARNING: This page is about a different car, the 2006 Suzuki Verona. However, it is still accessible from the selected car via links, so may be relevant.
The heater core heats the air before it enters the vehicle. Engine coolant is circulated through the core to heat the outside air passing over the fins of the core. The core is functional at all times and may be used to temper conditioned air in the A/C mode as well as in the heat or the vent modes.