Brake light circuits run in parallel. This means that if one bulb fails, the rest of the bulbs in the circuit can still illuminate. This is why when one bulb is out, that bulb is usually the best place 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 bad and needs to be replaced. If the bulb reads continuity or close to, the bulb is good and the socket and circuit need to be inspected 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 indicated. 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, the wiring and related circuit must be tested.