Some aftermarket radios have aux inputs that you can use. You could also open the radio and add it yourself assuming your past warranty. Most radios have one (or more) main baords for the radio/amp and the preamp for the tape/cd is mounted on the tape /cditsself as a seperate module. There will be one or more plugs from the tape or cd module to the main baord or amp board that will carry the power and signals as well as the line level audio. You could cut that wire and rig up the taoe/cd in switch to trick the amp into using that input.