Glow Plug Relay: Description
WARNING: This page is about a different car, the 2006 Dodge Sprinter. However, it is still accessible from the selected car via links, so may be relevant.
The glow plug relay supplies battery voltage to the glow plug through a timed cycle that is related to coolant temperature. The glow plug relay is located under the battery. The purpose of a glow plug system is to improve combustion for low emissions and to achieve easy starting at low temperatures.