Heater boxes . . . Some here have plumbed them directly in to the rear jump seat space, so the much shorter run gives better, hotter air, but maybe presents a real problem if you want to have air directly at your feet and/or up through defroster vents. My car has defroster vents, and the air runs in tubes under both rocker panels, feeds standard looking Porsche floor vents, and the defroster vents too. My approach is to insulate these tubes under the car with rigid Styrofoam, contoured over the pipes and then sprayed with undercoating. Fitting the Styrofoam is a bit of a pain, but it should help. The system at its finest is only an approximation to heat. l do rather little driving in cold weather.