i had a similar problem bout 6-8 months ago....there is a relay in the steering column, that connects the brake lights and the turn signals (connects is not the proper term, but you catch my drift) mine was stuck was not allowing the signal to get to the bulb....find this relay and shake it, what not.....took my about 150 bucks of diagnostic test and 3 days to find this one....pain in the butt.......but so simple to fix.........
does your third brake light still work? it should, it is on a different relay....if so, this is most likely your issue........hope that helps.....