To drive relays and small motors in automotives, the LIN bus is used. Examples are seat position, window lifters, mirrors, gears, rain and light sensors.
Frames in a LIN bus can be monitored with some Arduino projects. For eample, there is a discussion on intercepting the LIN bus of a Porsche, with links to a live demo on Vimeo and a document of Engine Control Units (ECUs) of Porsche
The whole project is open-source at zapta@github.
Here are some more references: