See if you can run a Core 2 Duo Mobile processor. the L2500 at 1.83GHz is more than enough for your CarPC needs. It sips 15W at most compared to a typcial 31W Mobile processor. They even offer ultra low voltage CPUs that sips 9W at 1.2GHz (hard to get). Therefore, look for a low power mobile processor if it works with your board.
I'm running an AMD Turion Mobile with 25W maximum consumption.