Something to consider is Audio Control's device at http://www.audiocontrol.com/caraudio.htm - called their "four.1i". It's an in-dash half DIN item with two sets of inputs, and one "master volume control" output. The thought is that you put two different sources into it, use the sources and the gain adjustments to set the levels you want, and afterwards the single knob on this four.1i product is your new volume control.

I've never used one, and am not really familiar with it. But that's the gist of it, something I'd suggest taking a look at and see if it does what you're looking for or gives any more ideas.