The brake light wiring system in most cars is not overly complex. At one end, you have the lights themselves: bulbs in sockets connected to a wiring harness. At the other end is the brake switch, where the pedal presses down and creates contact that completes the circuit. The system draws power from the car's battery.

The circuit is essentially the full run of wires from the battery to the fuse block, from the fuse block to the brake light switch and then from the switch to the brake lights themselves. Sometimes, the wires may also run to the body controller. Most of the wiring is bundled in a wiring harness that runs through the entire car.

A parallel circuit is used for brake lights because one bulb can burn out, and the other bulbs will still function normally. If brakes are wired in a series circuit when one bulb fails, other bulbs are rendered inoperable. If one bulb is burned out in a brake circuit, it is likely associated with the individual bulb or the socket.