A two-contact brake light bulb with a BAY15D base and an S25 envelope. It combines the position light and the brake light: one filament glows dimly, the other brightly when braking.
Replace the bulb when a filament burns out: the position light or the brake light does not work. Fit the same wattage rating as before. The BAY15D base has two contacts at different heights, so it fits in only one way.
