I know you can hook up an ipod to the car radio with the wire harness but I needed to know is there a separate harness that will allow you to use your mp3 with the car's radio. Thanks

Isnt a Mp3 player and an ipod the same thing??

http://www.rx8club.com/showthread.php?t=77790

The USB unit lets you use the 1/8" plug to connect or you can use a USB stick.

I think that there are other hookups (PIE)