you'll probably need the same Scosche VW01 wiring harness and Metra 40-EU10 antenna adapter to hook up the new head unit to the amp/speakers. You'll need RCA cables (with video... yellow, white, red) to hook up the XBox to the head unit. You're gonna have to power the XBox with a power inverter. The CD Changer should have come with some special Kenwood cables to connect it to the head unit.

as far as the hand brake, i'm guessing it receives power, ground, or similar when the brake is either enable or disabled. you're either going to have to supply it with constant ground or 12V to "fool" the deck into thinking the brake is enabled. or is it hooked up using a speed sensor? what does the deck's instructions say about hooking up the brake wire?

good luck!