Automotive brake light systems use parallel circuits. A parallel circuit is different than a serial circuit because if one bulb fails, the other bulbs still function normally. This is not true in a series circuit. In a series circuit, if one of the bulbs burns out the entire circuit is rendered inoperable.
In a parallel circuit if one bulb fails all the others still work. This is because in a parallel circuit current can flow through more than one branch at a time. If one of the branches fails and creates an open, the energy will travel to the next branch. Each bulb in the brake light circuit receives 12 volts regardless of how many bulbs are added.
Each item added to a parallel circuit decreases resistance, in turn increasing current. Current flows through all of the bulbs regardless of their resistance. Remember Ohm's Law (I=E/R) whenever working with electrical circuits. Always be careful and check specifications before adding additional items to a factory system. Adding additional items lowers resistance. This increases current (amperage) in the circuit and can damage components and wiring.