http://www.mp3-box.org/mp3-car-stereo-jack/

How do you hook up an MP3 player to a car stereo that has NO CASSETTE/CD PLAYER/AMP?
I have a basic MP3 player and want to play it through my car speakers. I saw several people do this, but they all hooked it up to the cassette or CD players, mine just has an AM/FM stereo. Can’t I just install an female jack and splice it into the speaker wires behind the stereo or something? Do I have to get an amp?
I already tried an FM tuner/transmitter. The first one I bought worked okay until I went to pull it out and it broke in half! The second one I got was a pain to use, it would barely come in on any frequency and I kept having to change the channel every couple minutes. It stopped working altogether after only a month… needless to say I’m fed up with those and I was wanting to hard wire something so I wouldn’t have to worry about radio interference and the like. And also without having to buy any more expensive electronics.
Just go to Best Buy and buy a stereo deck with an AUX jack, they can run $60-200+. Since your saying you want to splice wires im sure you can install a stereo.
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