I had tried several things, but finally found a great solution for playing my (non-Ipod) mp3 player though my factory stereo. I don't know if this will work for any other factory configuration, but I have a 2005 GT with Monsoon 6 disc head unit and no XM radio. I also have Onstar, if that matters - I'm not sure. I bought a PAC AAI-GM24 (which is discontinued by the mfg, but available on ebay). It allows you to add two aux inputs (and will also turn on remote devices if you want it to). You only have to toggle through the band functions (AM, FM1, FM2, CD..) only now I have XM1 and XM2 bands. You can even adjust the input level on each aux in to match the levels. Works great and sounds as good as my CD player!!!
