What brand is the head unit? Does it have the feature that displays track titles and data played over the radio. Its called RDS. If it has this, then you can go buy something like what I use. Kenwood makes an add on that plugs into the radio's antena and allows you to feed mp3s to dvd channels to it. My radio in my Z does not have RDS so I blindly messed with it till I got it on the right setting. I then used it to listen to my iPhone in my Z when the CD player quit working.
It sort of works like the FM modulators, but on a much larger, stronger scale. They run around 120-200 bucks.
EDIT: For my alltrac, I just bought a head unit that has an MP3 plug.
I have a scion TC radio left over I am going to list that will work in an alltrac, and has MP3 input and controls. I did not use is since I was not sure how to make system 10 work with it, and cause I would have to modify my bezel. If your interested in it, let me know as it would be a fairly cheap option to get MP3s in your car.