The ballasts do not go into the housing, only the bulbs do. I have always mounted my ballasts (four them, two for low beams and two for highs) behind the front grille. Plenty of air flow through there and will never over heat.
HID 'conversion' kits are nothing more than a ballast and bulb. Disconnect your old bulb, plug the truck side wires into your ballast, install new HID bulb into headlights and PRESTO! Then mount your ballast where you want and make it look pretty. It does get a little more 'complicated' if the there is only one bulb for high and low vs two separate bulbs.