If you want to go the cheaper route you can try my method. I purchased an FM transmitter that plugs into the cigarette outlet on the dash and attaches to your mp3 player (iPod in my case) through the headphone jack. The one I purchased also doubles as an iPod charger. I believe I spent $12.99 on eBay altogether for the item and it works well.

I will tell you that in order to have decent volume with it I have to turn the volume up to around 20 and up to get decent sound (this depends on the quality and loudness of the mp3 file I am listening to). For example if I have an mp3 that is on the quiet side I may have to turn the volume as high as 35 to get it loud enough... and if this is the case the feedback from the FM frequency can be heard in the background. Considering the Crossfire's stock head unit only goes up to volume 40 (I always thought this was a random number to stop) this can be problematic. Furthermore the FM frequency can be disrupted by various exterior objects temporarily. I also sometimes notice if I am starting the engine cold I don't have to turn the volume up as loud to get good sound, but when the car has been running a while I need to turn the volume up higher to have the music loud enough (strange I know, but thats how it is).

If you have the cash, you should upgrade to a head unit with a jack for mp3 players. But if you don't feel like spending the money, my option is a cheap alternative which is certainly a decent option -- but you get what you pay for.

Note: I also have a garmin GPS and I bought an AC (cigarette lighter) jack four way splitter on ebay for around $8 which I plug into the cigarette lighter and run out of sight under the driver side seat. It runs both the mp3 jack and the garmin jack just fine.

Someone else on the forum can help you with other alternatives and you should also use the search function on the forum as this has been discussed fairly extensively.