If the brake light is the same bulb that is for the signal, check if the rear signals work. If they do, you know all the wiring to those bulbs is okay, and the bulbs are okay. That leaves the switches inside the signal switch, or the wire going up the steering column to the signal switch.
If the rear signal and the brake light are separate bulbs, especially when the rear signal uses a yellow bulb instead of the normal red one, all three brake lights are tied together on one common circuit. Since the center light works, you know everything is okay up to that point. The most common suspect then would be the ground circuit for the two side bulbs. A less-common cause would be a break in the splice or wire between the center bulb and the two rear ones.
Monday, June 8th, 2015 AT 10:56 PM