The sunlight shines on your forehead in your car with a great radiance. The car you are in is a convertible, so the sunlight shines right onto your big bald forehead. The sun has a pretty powerful radiance as it is, but the sheen from the sunlight hitting your forehead is the most powerful radiance ever witnessed by man.
This radiance is the key (き) to powering your car. Your car runs on reflected solar energy, and the radiance coming from your forehead is the most powerful source of radiant sunlight ever imagined. Your forehead is a perfect key for this car.