Does the signal still light work? If so then possible/more likely the multifunction switch is defective!! If signal light DOESN'T work either, check in trunk for a wire problem, if none found, replace the bulb socket. Brake light circuits go through the sig. light (multi) switch before going to rear lights and either left, right or both can be inop. if the switch is defective! Weird I know.
Yes i know what you meant! However the most likely suspect is the signal light switch which is part of the multifunction switch! The sig. and BRAKE lights both share the same circuit, so if sig. works so should brake! THIS SWITCH CONTROLS THE 'BRAKE' LIGHTS ALSO and either brake light can be affected!! Get a mechanic to check it out, and replace this switch I am talking about!!