With electronics, you often transport data over a wire. Famous protocols for using only a few wires are SPI and I2C.

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:

The Specs

LIN monitoring

Discussions and projects

Datasheets and open notes