Check the loom of wires inside the trunk, by the driver's side trunk hinge. Open up that loom and look for frayed or broken wires. Repair as necessary. That loom is exposed to constant flexing as the trunk is constantly opened and closed. Eventually, some of the wires inside that loom break. I would note the color of the wire(s) for the back lights, by looking near the bulb socket. Pay particular attention to that color wire inside the loom.