The reverse light switch is a sensor screwed directly into the transmission (gearbox) housing. It detects when the driver shifts the gear selector into “Reverse.” Upon engagement, it completes a circuit that turns on the white backup lights at the rear of the car and activates the rearview camera or parking sensors. If this switch fails, other drivers won’t know you are backing up. It is a common failure point on manual transmissions due to the constant mechanical wear of shifting gears.