defo not the wiring, (i rewired it all last year) :D
now the switch does work, as when you put in diff lock the light comes on bright, and when you take it out the light goes out. but when your driving it slowly starts to come on, dim at first and slowly getting brighter, but not to full brightness. almost as if the mechinism is creeping just to the point of the switch erm switching, instead of being held at one end of its travel or the other by the diaphram accuator thing.
but iam not sure if it accually locks the diff as when the light starts to come on iam driving.
i think i need to jack up the front end when the light starts to come on and see if it is locked or not.
iam thinking of binning the vacuum system adn fitting a linkage of some sort.