An Overview of a Lighting Circuit
In many vehicles, the electrical system is broken down into multiple circuits. The reason behind this is a combination of simplifying the electrical system, simplifying the troubleshooting process, faster and cheaper factory installation process, and being able to reduce cost and weight by eliminating extra wires and cable management products (i.e. harnesses).
A lighting circuit is generally composed of headlights, taillights, running lights, and interior lights. Breaking down the electrical system into this group of devices simplifies the troubleshooting process when a light stops working. You will still have to find the specific circuit for that light, but you still won't have to worry about electrical components outside the lighting system from getting in the way. The headlights and taillights are part of the lighting system, but each operates on a separate circuit within the lighting system.