Yeah, you should be able to get a DC to DC converter that basically hooks right up to the battery. Like an oversized laptop battery you can put it to sleep and wake it just like would at home. Down side to this that I've heard is if you aren't going to be driving the car for a while (two or three days in worst cases) it drains the battery down to far so the car doesn't start. It would just be best to shut down the computer every night and start it up every morning, I know I'd be too lazy for that one. best of luck.
2.4gHz 15" MacBook Pro, 1.66gHz Core Duo Mac Mini, 2.5gHz G5 QUAD, 733mHz Quicksilver, 450mHz G4 Cube, 700mHz G3 iBook, 350mHz Sawtooth G4, 350mHz Revs. A and B B&W G3, 16mHz Powerbook 100, 8mHz Macitosh Classic.