If there isn't another input, you can put a three way switch into the the RCAs that supply the changer output to the headunit. A CD is still playing in the changer, but the headunit gets input from the MP3 player. Don't know if you can buy one, but building one isn't hard with Radio Shack parts. May not be the ideal way of doing it, but it works well if you don't want to invest in more equipment.
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