For 12v, you just need a 12v regulator.
For 5v, you can use a POL power supply.
Another option is a Carnetix P2140, but this is decidedly overkill for what you want.
I searched the forums to see if there is a clear answer to this, but did not find a definitive answer. I currently have my PC in the trunk of my 2009 CobaltSS and the screen and the USB hub in the front. The Screen and the hub is powered by the PC power supply, but the voltage drop in too large. I want to put a separate supply in front for the LCD panel and the hub. I am considering the picoPSU-60WI or the M2-ATX, but I dont know if the load of the screen and hub would be enough to turn on the supply. Would either of these work? What is everyone else using? Of course I would prefer a simple power supply that supplies regulated 12V and 5V from the 12V from the car battery (without the ATX connector), but I cannot find such an animal.