I work on shit like this for a living. Without any wiring diagrams handy it sounds to me like the lamp driver module is going out. Which is probably the entire body control module on that car. Sounds to me like you have done your diligence on the lamps circuits in the rear. You will at least need to take it to someone that can scan and confirm the module is going out by properly checking the wires from the module to the rear lights. If your car does need a module it will most likely need to be programmed by the dealer in most cases.