you need to get a test light and see if current is right at the bulb socket on the rear brake lights.
If yes, there is current, than you have bad bulbs or a very bad contact in the socket itself.
If no, then you must have a break in the wiring. So you'll have to follow the wiring on each side of the trunk to some coupler or place where you might see a break in the line.
It might be possible for you to splice into the third brake light as an emergency measure but that might also give you dim bulbs, I don't know for sure.