there are 2 ways of doing this. The better more expensive way is:
power supply (like carnetix) w/ builtin startup mod, so you wouldn't need webmaster's auto power on mod.
the cheaper way (the way i do it) is:
get a cheap laptop car adapter on ebay, hack it up and connect that to the battery along with the xenarc, and get the auto startup mod. (of course connect the screen and adapter to a relay to turn them on when the car turns on).
Another way is use an inverter to power the laptop.