demoninvictus

i can't even think of the damn thing, but i know for a fact that there are adapters for alpine decks that you can throw on that allow you to hook up tv's, game system's audio tracks or even a dvd player's audio feed.. well i need to find one that will support audio input from a stereo jack... or even just a normal headphone jack sized input... but i don't know what they call these things? anybody have any clue what i'm talking about? me and my buddy had a hair brained skeem for those long road trips to hook his acoustical guitar up to my head unit so we had something new and fresh to have fun with . any suggestions guys? thanks
peace