QUOTE(j_guzzler @ Jul 28 2005, 06:40 AM)
The sad thing is that you need to convert it from DC to AC then the xbox power supply converts the AC back to DC, stupid but the cheapest way to do it without changing out the xbox power supply with a DC to DC transformer.
This thing ( http://mini-itx.com/store/?c=10#p1818
) looks like it might work? It's ATX, so it would need a few diodes to drop the supply standby power for a 1.5 or less XBox, but other than that is seems to do the job. Additional bonus, it would seem to be capable of switching the XBox on after you start the engine, and off when you leave the car switched off, automagically. Because the ATX cable plugs into it, you wouldn't need to mangle the power supply itself.
It would also appear that you could fit this inside the XBox case, keeping the XBox looking tidy, and still usable in the home (albeit with a 12v power adaptor).
To customise XBMC for the car, the place I'd start would be keymap.xml, rebinding the gamepad to behave more like the remote, unmapping less-used things in favour of adding shortcuts to music/videos, skip track, stop, etc.
Possibly the best control method might be a remote control rather than a pad, maybe combining an XBox remote and dongle to create a USB remote: put the IR diode on a long lead and hot-glue it to the IR receiver, then use a Zener diode or voltage regulator instead of batteries to power the remote. Hook up a USB port in your dash, near the LCD screen. If appropriate, rewire the buttons. More advanced version: use a USB PIC to emulate the XBox remote, and a custom keypad.