Technical High Level (Centre) Brake light problem

[Lavatrax]

I'm chasing a problem with the centre brake light. Initially I thought it was just the bulbs as they were all blackened. So replaced all 8 bulbs but no joy. The connectors for the bulbs are clean, not oxidised and I can't see any broken wires in the rubber connector to the tailgate. I've tried manipulating the wires while the brakes are on but can't even get the light to flicker.
Are there any other points of failure to check?

Cheers

 

[DGPunto]

Did you check the earth. Also do you have a test meter? If so check the power to the bulbs/light fitting. Suspect it is a wire problem but check first before you go cutting the wiring