had the same problem - warning light comes on when applying brakes -- rear lamps all worked !!
A few days later the culprit bulb failed. Could be a bulb that is developing a higher than normal filament resistance as it 'prepares' to fail.
you can remove all bulb one by one and tap each one in your hand (exert some mechanical shock) and the culprit bulb will surely drop the filament in response ...
just my 0000.02's worth