You could use a voltage inverter (to get A/C voltage) and use a home amp with volume control. Point is, with car stereo amps...they are only amps. Don't have remote volume control (yes you can manually adjust the gain...lame) and they don't have input selectors. The head unit in a car acts as the input selector and volume control.
On a home amp, all of this is combined into one unit. But of course, they aren't as compact.
Your MB must already have a decent head unit...still don't know why you can't use the speakers already in your car, and the amps already in your car and your head unit, and just add the XBOX, but you haven't really described the goal of your system. In your last message you made it sound like you just want captives in your trunk to use it.
