Brake Light Circuits
Brake light circuits are parallel circuits. A parallel circuit is used because one bulb can burn out, and the other will still function normally. If brakes are wired in a series circuit when one bulb fails, other bulbs are rendered inoperable as well. If one bulb is burned out in a brake circuit, it is likely associated with the individual bulb or the socket.
Each item added to a parallel circuit decreases the resistance in the circuit. This increases the current. Current flows through all of the bulbs regardless of their resistance. Remember Ohm's Law (I=V/R) whenever working with automotive circuits. Be careful and check specifications before adding additional items to a factory system. Adding items lowers resistance and increases the current (amperage). This additional current can damage components and burn sensitive wiring.