brake lights on landcruiser J100?

My brake lights do not work! thats the Lh/Rh and roof level lights, there is no electricity when brake pedal is depressed (nor on the trailer socket)
Please help...MOT is due
I have taken out the light clusters-all plastic so no usual earthing probs, changed switch on brake pedal, obviously checked ALL bulbs.
I can only think the problem is in the tow-bar light socket BUT where do I start?
Cheers

Bob