Not impossible, just difficult. Last night I took the glove box out and succeeded in injecting sound into the system through the CD changer's connector as a test. I also had the dealer fax me the schematics of how the modules interconnect. BTW the changer is clearly marked "Pioneer" on the outside label.

I agree that controlling third-party gear through the factory head will probably never happen. But feeding sound into the system is do-able.

There are two inputs to the amp, one from the CD changer and the other from the radio/tape head. Both use two sets of shielded twisted-pairs carrying differential signals.

Originally I hoped to feed audio in through the CD changer's wires. But the system is too smart. The only time it listens to the CD inputs are when a disc is present and spinning. With the changer removed or even with the 6-pack removed you can't even select "CD" on the head unit. The same apparently goes for the tape unit. But you can always select the radio.

So my plan is to locate the harness between the head unit and the amp and tap in at that point. I'll add a small shielded box containing two DPDT relays to switch between the signal from the head unit and the 4 volt output from the empeg. I'll experiment to see if I can feed the empeg signals directly into the amp. I may have to attenuate them and possibly add some isolation to convert from single-ended to differential.

This is actually the easy part of the job. Physically I'll probably have to mount the empeg under the passenger seat. So my plan is to adapt or clone the display server software to send screen images to a microcontroller driving an LCD on the dash. Control will be via the Sony stalk-mounted remote. This is feasible - I have done the same in the past to mirror the screen of an HP palmtop computer on a PC's screen.

If I can pull all this off I suspect it will be the most elaborate empeg install yet.