What radio do you have now?
Most aftermarket radios has an AUX selection. ANything plugged into that is usually with RCA connections. You can get then an RCA-->3.5mm stereo plug. Get an extension cable and it can plug into your ipod.
Unless you really want to control your ipod from the radio, which is a royal PITA and not very good either. Most folks just control the ipod from the ipod itself.