PC in the car?

Have I ever asked a question I play with the thoughts of building my little PC and putting it all in a small wooden tower (built yourself)….. then place somewhere in the trunk and use the PC as an MP3 player and as a navi…. my question I take a voltage converter or a 12V power supply? If where are there such PC power supplies for 12V running in the car? Still how can I connect the car radio to the PC to play music? Or how can I do that?