'04 Audio Interface

A bit of short background. All I've wanted from the day I bought my Prius is a good way to tie in my iPod. FM transmitters suck. Anyway, a year ago, I bought a DICE. Within a month of installing it, my MFD broke. I was barely out of warranty but still managed to get it replaced for free. I ran with the new MFD a couple of weeks, then reinstalled the DICE. Within days, my brand new display had completely locked up on me. I removed the DICE and the display started working again. It hasn't failed me since, so I could have just had a loose connection, but now I am afraid to plug the DICE back in, lest I break my $2500 MFD.

So, being an electrical engineer, I set out to see what I could do to hack together my own audio interface that didn't waste time or risk breaking things by messing with communication. I reverse-engineered the DICE and found that it sends audio left, right, and common to the radio. I found 12V in, ground, a 100 ohm resistor from ground to audio signal common, and some other traces I didn't bother to follow. The circuit seems to run off an ordinary 7805. There is a 5V differential bus transceiver that presumably talks to the car audio system.

The bad news is, it appears as if the audio lines will be ignored unless I can trick the car into thinking there is a CD changer plugged in, and it doesn't look as if I can do that without talking to the car.

The good news is, it doesn't look to me as if there's anything in here that could be harmful. I suppose maybe if the ground lead were to become disconnected, the circuit could dump 12V onto the comm lines? But then it probably would have broken more than just the display. I powered it up on my bench and nothing seemed to exceed 5V (but with only a DMM, it's hard to be sure). Is it possible that the Prius runs at 3.3V and only barely tolerates 5V signals? I plan to check this tomorrow at work, if I can borrow a battery-powered oscilloscope.

Any thoughts/advice?

Thanks!
-Ben