there is an auxilary jack next to the 12v jack above the glove box. all you need to do is go to best buy, circuit city, radio shack, etc and pick up a male to male connector that fits from the heaphone jack in your mp3 player to the aux jack. onl costs around 8 bucks. sounds great.
I'm thinking of getting the Archos MP3 player that has 20GB of space for MP3s. I can pretty much fit 300 hrs of music in there. I did order the iPod but they were on a pretty substantial backorder, but my local Circuit City had these Archos readily available. Still debating, though.