Only simple stuff. Have you put a meter across the connector to the switch to check for continuity when it is on?, i.e . is it being properly actuated in the box?
Then have you checked there is power to the bulb? Ditto the switch?
Is the bulb holder clean and the spring springy enough to ensure a good connection?
With each wire have you checked for continuity at both ends? Ditto short to earth?
Having looked at the wiring diagram, there is a connection to the BCU so it has a role in all this, how much of a role I do not know, at which point apart form saying good luck, I'll leave it to others. At least it isn't an MOT failure! (Page 43.1 on Rave manual for circuit diagram)