Brake light circuits run in parallel. In a parallel circuit, if one bulb fails, the rest of the bulbs in the circuit can still illuminate. That's why when one bulb is out, the bulb and socket are usually the best places to begin diagnosis.
Brake Bulb Inspection
Most of the time, it's apparent the bulb is bad by the discoloring inside the bulb. If not, remove the bulb and test it with a multimeter set to ohms. Place the negative tip on the bulbs case and the positive tip on the contact. If the meter reads OL or infinity, the bulb is burned and needs replacement. If the bulb reads continuity or close to, the bulb is good, and the socket and circuit should be tested next.
Place the bulb back in the socket and use a pedal depressor or have someone depress the pedal for you. Slightly wiggle the bulb around in the socket while the pedal is depressed. If the bulb flickers on and off, a loose socket is at fault. If the bulb is still not illuminated, measure voltage in the socket by placing the positive probe on the brass tab and placing the negative probe on the inside of the socket. Battery voltage should be present. If not, test the wiring and the related circuit.