The first and most obvious cause of brake light failure is a loose or broken bulb. This can be fixed easily and cheaply, and it is as simple as screwing in a light bulb. Bulbs can become loose over time or in the course of normal wear and tear. So before you go exploring your vehicle's electrical system for faults, check to make sure the bulbs are in place and unbroken.
Another cause of brake light failure is a poor connection between your brake, the application of which should trigger the brake light, and the light itself. Repairing this is more complicated than checking the bulbs, and is best left to a professional.
Every electrical apparatus on your car is powered through a fuse, and just like a blown fuse at home, a blown fuse in the car can leave whatever is wired to it in the dark. Your car's owner's manual should illustrate where your fuses are. So find out which one controls the brake light, and check to see whether it is blown.
