Climate Control: Radiator
WARNING: This page does not describe the selected car, but rather 41 other vehicles, including the 2003 BMW M5, 2003 BMW M3, 2003 BMW 540i, 2003 BMW 530i, and 2003 BMW 525i. However, it is still accessible from the selected car via links, so may be relevant.
The radiator is used to dissipate the excess heat from the hot coolant. At operating temperature, some of the engine coolant circulates through the radiator and gives up its heat to the outside air, as it passes across the radiator. So the radiator is a "heat exchanger."