Disco 2 Brake light problem - third light only & taillights go off when braking

Someone pointed out to me that my taillights completely switch off when I brake, and only the third brake light on the tailgate screen is illuminated, indicators are unaffected. I'm used to earth problems causing other lights to dim when another one illuminates, say tail lights dimming when the indicator flashes, but this is completely killing the tail lights when the brakes go on, and indicators are unaffected. Has anyone any ideas on where to start with this?

Start with the towbar socket, check for corrosion, the LH and RH tail lamps have different earth points while the RH brake lamp shares the earth with the high mounted one if you see what i mean