Glow Plug: Description
WARNING: This page is about a different car, the 2006 Jeep Grand Cherokee. However, it is still accessible from the selected car via links, so may be relevant.
Glow plugs are used to help start a cold or cool engine. See Fig 1. The glow plugs will heat up and glow to heat the combustion chamber of each cylinder. An individual glow plug is used for each cylinder. Each glow plug is threaded into the side of the cylinder head below the cylinder head cover.