I think mounting the microphone close as possible to the listeners ears would be best.

But would require subtracting the music from the input signal, as Paul said. And as Toby correctly pointed out, there are a lot of tricks you need to do in order for that to actually work. Simply inverting the signal is not enough.

I also do not want the microphone to pick up conversations in the vehicle and then attempt to increase the volume over the conversations, thus causing a different kind of feedback loop.

Anyway, I'm looking for a simple project, and anything involving noise cancellation or signal subtraction would be extreme overkill for what I want to do. I merely want the average of the microphone input level to control the amount of volume attenuation.