I have a a few problems with this - hope someone on here may have a few ideas...
Firstly, it's all in Japanese which doesnt help with the second problem ;)
The unit seems to be able to communicate with the multi-changer in the boot - it will change CD's when told to and the display shows it happening, but it seems to have stopped playing them; I am not certain if it just isnt playing them or if the output has been muted in some fashion - the audio on the radio and TV still tuner works. The internal CD player seems to suffer from the same problem...
There is what seems to be an error message on the display (E03), but not having the book (or the ability to read Japanese) makes it a bit hard to diagnose ;)
Picture of the unit and error message here if it helps;