In the trunk do you have 12 volts at the brake light sockets? All the sockets dead at the same time would indicate a main feed wire is dead. I would open the harness connectors and trace from where you have power and work back towards the rear. It is either a broken wire or a bad connector pin in one of the connectors. As a last resort you could run a dedicated brake light wire from where you do have power all the way back to the brake light splice in the trunk. At least you will have brake lights while you try and find the actual problem.