well, since a CD changer from the other cars would not work in the DSP car, how about trying the questionable CD changer in the non-DSP car? If I remember correctly, the CD changer should have both coax and 6-pin connectors for the sound - now, since there are dozens of CD changer models out there, I might be wrong in your particular situation...

I would suspect, that the CD changer is defective - probably something with the laser head unit...

I think I've heard somewhere on the forum, that there are repair facilities that can repair the changer, the radio, the nav for relatively little money...