Cabin Heater: Description
WARNING: This page is about a different car, the 2005 Jeep Liberty. However, it is still accessible from the selected car via links, so may be relevant.
The diesel engines uses a supplemental cabin heater which is an engine mounted mechanical device commonly called a viscous heater. The viscous heater is used to help heat engine coolant prior to it entering the heater core. The viscous heater is driven by the engine accessory drive belt and has an electro-mechanical clutch which is controlled by the engine control module (ECM).