I built my first Car PC in 2001. A set-top PC, a game-pad, a tweaked Windows install, and some WinAMP plugins provided me with a much more capable MP3 player than anything on the market at the time. I continued to enhance this machine until retiring it in 2009 after the video-out failed, and my cell phone was able to provide similar functionality.
I built my second car PC for the DeLorean, using all the software and hardware capabilities that came about since my first car PC. I presented this platform at PhreakNIC 9. My presentation downloadable here.
I added a multi-function rear-view mirror to my DeLorean. This added automatic, electrochromatic dimming, a digital compass, an outside temperature display, and Homelink (garage door / gate remote) buttons, all with a single upgrade to a part that was already broken in my vehicle. I've been very pleased with the results. While I installed this, I documented the procedure and took photos. Although my instructions are specific to the DeLorean, my instructions may be useful to anyone considering such a project.