The brake light switch is a small, spring-loaded plunger switch mounted on the brake pedal arm. When you press the brake pedal, the plunger extends, completing the circuit to illuminate the rear brake lights. Beyond just lights, this switch is a critical safety interlock: in automatic cars, it signals the transmission to allow you to shift out of “Park,” and in push-to-start cars, it confirms the brake is held before the engine will start. If it fails, your brake lights won’t work, and you may be unable to shift gears.