I like the Lind DC to DC power adapters designed for autos. I use an automotive sytle relay latched by accessory power to cut fused 12Vs to the adapter. This then turns off the power to the DC to DC power adapter when you turn off the accessory power.
The laptop will still be powered by its' internal battery (but won't drain the car battery).
check out
http://www.lindelectronics.com
Neil