Hi Nigel I think they all have a bulb monitoring system, its built into the back of the rev counter, although not sure if fog lamps go through this. I have by-passed one for a friend who had a rear sidelamp inop, but I dont think they are that expensive, and are easy to fit. If this unit failed however, i think you would still have power to the switch, but i don't have a wiring diagram so I could be wrong.