Just a stab in the dark - 3rd brake light? the one in the tailgate? If so the wiring loom where it transitions between the body and the tailgate is always worth a look for frayed/damaged wires either shorting to earth or to each other caused by the constant flexing during opening and closing of the tailgate and aging of the insulating of the wires making it brittle. Although maybe not so likely with your symptoms. The other thing can be earthing problems on the rear light clusters. Lots of "funny" symptoms can occur with this. Try turning on the sidelights and, with ign switched on, press the brake pedal. what are the lights doing now? Unexpected results are often due to poor earths. Then again with all these things being area networked via ecu's these days you might get a DTC displayed if you can find someone with VCDS to interrogate it for you.