I need to replace the taillight/brake light on my car, and it's a bulb with two filaments (known as 12V P21/5W - see pic)
My car has two taillights on each side. A standard 5W bulb for the top half, and the bottom one also doubles as a brake light when you use the brakes. So if you have the lights on, it lights up like the top half of the taillight (a normal 5W bulb) but when you press the brakes, it also lights up the other filament (maybe both at the same time? I'm not sure)
My question is, since it's connected like a normal light bulb, how does it know which filament to light up? Does it depend on how much power is provided?