Greetings!

As for cancelling the music to avoid feedback, what you might be able to do is the following. Take a copy of your regular signal, as seen from your amp, and invert it. Add it to the input from the microphone. You would need to have some kind of adjustment for calibrating the cancellation circuit - perhaps just a amplifier control and calibrating the music level when there is no noise in the car (parked). Might work...