Using a switch to toggle between listening to the empeg or the bose would be just fine. I'd prolly put it in the armrest with the climate controls. Since I'm fairly new to car audio, I don't know what type of switch I'd need. You mention different voltages between the empeg and the bose... does the amp auto-detect or did you have to put something inline in your setup to compensate? As far as the switch goes, is that something special or would wiring it up through a relay from Radio Shack work?

For the climate controls, if the faceplate doesn't come off, then I think what I'll do is cut a slot in the side wall inside the armrest and use the faceplate to support the control panel. That's about the least notable modification that I can think of.