Head Units for Car MP3 Players?

dotcomian asks: "The idea of hooking our iPod to our car's audio system so far is our best shot for musics on the go in cars, however, it requires a double investment to the head unit. I'm wondering if someone has a better way to replace cd-changers with MP3 jukeboxes, the head units would be the controller and/or a CD player. I've found several websites from google's search result, but nothing is close to what I have in mind. Any ideas?"