Cool, the fake_tuner was helpful in finding the problem. I ended up getting a DB9 gender changer just so I can debug future in-car issues. Seems to be working fine.

The volume spike actually wasn't related to the volume boost stuff. It was my own stupid bug.

I am going to put up a new thread in the Programming forum to discuss the audio overlay thing, I have a few thoughts on how we can do it better. There are some weaknesses in its current incarnation.