If your steroe has an AUX jack, you can just use an AUX cable like I do. It plugs into the headphone jack on the iPhone, and it plays through the AUX input. You can't control the iPhone with the stereo, but I don't care about that personally.
If you do a search, there's several places that show how to mod your stereo to have an AUX in jack if it doesn't come with one already on the faceplate. Also several sites show how to mod your stereo to include an iPhone adapter. Can't remember any of them, since I require any car stereo I have to have an external AUX jack on the faceplate for the reasons I stated above, but if you search Google you'll find it.