Hello Stephen,
Can we take it you have checked for good earths at the rear brake light units?
If good:- You say you have checked all wiring for breaks - including stripping back the flexible rubber sleeving section where the wiring harness passes from the body, into the boot/hatch? Favourite place for breaks/partial breaks due to years of flexing, so always a suspect area.
Could you let us know how you get on?
Kind regards,
Gareth.
p.s. Double-time charges may apply for Sunday advice!