You can use the aux input, however you'd still need some means of playing MP3 files into that input line. You can rig up a phono to RCA line with Radio Shack parts and attach it to an MP3 player, or if you want to get hardcore, you can wire in a computer (which is a son'ofa'bitch to do right)

The last time I wanted to play MP3's in my car, I just sat my laptop on the passenger seat and let it play. h: