Hackaday seems to have had a run of car hacks,
http://hackaday.com/2010/03/16/adding-an-automotive-cold-air-intake/
This one looks really interesting and the arudino means you could take it quiet far http://hackaday.com/2010/03/16/keep-tabs-on-your-car-without-obd-ii/
http://hackaday.com/2010/03/16/follow-up-hacking-onstar/
http://hackaday.com/2010/03/16/adding-an-automotive-cold-air-intake/
This one looks really interesting and the arudino means you could take it quiet far http://hackaday.com/2010/03/16/keep-tabs-on-your-car-without-obd-ii/
http://hackaday.com/2010/03/16/follow-up-hacking-onstar/