You could use an auxmod for your MP3 player, it connects to where a cassette deck/MD would go. This way you can use both CD changer port and the cassette deck port.
Alternatively, if you have some soldering abilities and electronics knowledge, just cut off one connector, splice the wires of the other connector and join both cables together onto one connector. Also put a multipole switch in the place where the wires split to select between two audio sources.