Well, if you can live with your MP3s coming from the front speakers only, it is possible to use the NAVI System audio inputs to the radio. If you apply +12V to pin 14 of the main connector of the head unit, the front speakers will mute the radio audio and instead present any audio signal applied to pins 12 (right) and 13 (left). Pin 11 is ground for these signals. If you have the NAVI, you obviously would have to switch these inputs somehow (e.g., a relay and a diode or two could be used to automatically switch the audio between the external input and the NAVI computer, giving the latter priority. Note however that it might be dangerous to drive an inductive load like a relay from the 12V mute signal from the NAVI computer--best to use some kind of buffer.)
There are two things to note about such a setup:
1) The radio volume control does not affect the NAVI inputs, so your external source will need its own volume control.
2) You will continue to hear the radio in the rear speakers (they are *not* muted by pin 14), although because of #1, you can just turn the volume down.
Note: several people have suggested that the NAVI mute signal merely reduces the volume of the radio. This is not true. The front speakers are completely muted and the rear speakers are unaffected. It sounds like it a volume reduction, but you really just hearing the rear speakers at full volume. (If you don't believe me, play with your "fade" control.)
[This message has been edited by avanti (edited 10-13-2001).]