mine failed MOT last month on this
problem - i checked the fuse - fine, then proceeded to take the switch out of the dash to check connections etc. Followed Haynes manual but still managed to break the switch - the bit you twist round just comes off now. But I got the front fogs working - must have been a dodgy connection? Anyway, if you start messing with the switch, be careful - there's a tiny lug you need to move to one side - if you look from below at the bit that twists, you'll see a little hole that you need to poke a tiny screwdriver in to move a lug that snaps really easy if you're too heavy handed. Take your time......