Brake Light Circuits
Taillight assemblies include circuits for the parking, brake, turn and hazard lights. Some vehicle's have a bulb dedicated to each taillight function. These are called three bulb taillight systems. Each side of the vehicle has a bulb dedicated for the turning circuit, one for the brake, and one for the running lights. Most taillight assemblies use multipurpose dual filament bulbs. These are called two bulb systems. These bulbs have a high and a low intensity filament. The high intensity filament is shared by the hazard, turning and brake light circuits.
It's important for a technician to understand that on these systems, the turn signal and hazard switch share the same circuit as the brake lights. These turn signal switches are a complex series of contacts. When these contacts get old and burn, the end result is brake light failure.