I do not know about cars, but buses are about the (electrically) noisiest environment you can imagine. You will need to place your stuff in a steel enclosure (big enough to disappate the heat) and mount ferrite beads on every wire in and out. Also you will need to power from a large 24v capacitor fed through a diode from the battery to maintain a reasonable voltage when the 24v drops during e.g. start.
OOPS, I'm talking 24V (bus), in a car it is 12 but the same should apply.

TI has some cute and economical DC/DC converters such as PT6302. Using a switcher really keeps the heat down, which is important when working inside an enclosure.

Have fun,

Erik