Brake lights not working properly

My first thought is you have a bad ground for the lights. They ground at a point behind the left kick panel down by the drivers feet. However there are a few splices in the line. The easiest way to verify this as the issue would be to remove one of the rear lights and run a jumper wire from the black wire to a good ground. If the problem goes away then you could remove the kick panel trim and start removing the ground wires and testing them to find the bad one. With all of the splices it could be anywhere along the system.