Why would you add liquid cooling? Going to overclock the CarPC?
In my opinion, liquid cooling in a car is just another thing that can go wrong during install, and handeling.
What if you run a bit low on water, and make a hard turn... During this hard turn the pump will draw air into the system, making cavitation bubbles, redusing the cooling effect.... This happens a few more times, and after a while, you have a system that either.. just shuts down (intel) or burn up (amd). Best outcome, you need to bleed the system every now and then.
You allso need to add anti-freeze in the coolant, wich again reduce the cooling effect AND can make corrodation (or how ever it's spelled), and all of a sudden, you have a leak in the system.. that can possibly damage all of your gear for the CarPC.
The Carnetix will be able to power the cooling system, but I have doubt about it delivering enough power for both computer, witch accessories and the liquid cooler.
As a side note... I have nothing against liquid cooling, on the conterary... every stationaried computer should be liquid cooled in some way