cut it and put a splice in (extend the wire if need be! dont be gash and just tape it!lol
it wont be a short i dont think this sounds like an earth fault. if its when u brake, it sounds like a poor earth on rear lights and finding earth elsewhere ie fronts, which will make them come on momentarily! double check the rear earths! do a continuity check if u can get hold of a multi meter!
Failing this, check any main earths in the engine bay! will be a main one from engine to chassis and gearbox etc.hope this helps