YOu need yourself a DC-DC power unit. Check out the Power Supply forum for information and reviews. You'll have to determine your max power needs but I think an Opus could work for you.
I have a Dell M70 that I have yanked the motherboard out of and have been searching threads for clear details on providing power to it. Money is no object and I want it to be seemless (ie: no bulky power inverter and when the car turns on the system comes out of standby). Any help is appreciated.
2006 Accord VP Sedan
alittle more and you can buy a proper dc to dc with startup/shudown controller. That way you can power other devices as well.
Car : 00' Chrysler 300M
Speakers : Rainbow Audio SLC 265 | Subs : Diamond Audio D6 12" Subs X2
Amp's : Hifonics ZXi 6006 AB | Hifonics BXi 1606 D
CarPC: HP Laptop AMD Sempron 3000+ | 1.5GB RAM | DVD+RW | 80GB 5400RPM HD | ATI R200 Video