you have a few options,
fist, how are you going to power it? an inverter? or a incar laptop charger?
you could just put a switch on those, and set your laptops bios to boot on power resume.
you could also set to power on keyboard so when you press a key it poweres up, or just open it up and solder wires onto the switch contacts and put a momentory switch somewhere that you can get to it.