The geek in me also looks forward to seeing how you do this. Would love to hear more details on your plan.

My solution was simply to replace the factory cassette deck with a bluetooth capable head unit. I just feed music to it right from my phone, but then I also have the option for using mp3 cds, the radio, and other aux-in devices.

I know that's not what you're going for, but I wanted to keep mine simple. Plus, car stereos are SO much cheaper than they used to be. Even for halfways decent brand-name stuff. My cheap-o Pioneer sounds great to me.

Good luck!!