I have an 06 w/o the mp3 single cd, I bought a product from peripheral that allowed to connect the ipod to the stereo and as a bonus i got the HD reciver also. everything is controlled through the sotck head unit, I love it. it dose not specify an 04 but i dont think there is a diffrence between and 04 and 05 so it should work, best bet is to check with the dealer and see what part number the 04 is and what part number and 05 is that should help you out, ONE thing i forgot to mention the headunit must have an AUX button.
here is the link
http://www.discountcarstereo.com/detail.aspx?ID=1213