if your mechanic is any good he should be able to fault find this problem. is there power going to the switch? if not check the fuse, wiring and relay. if there is power at the switch is there power going to the rear fog light? is the a good earth/ground connection on the rear fog light?
Thanks Montego - for the welcome, and for the hints.
There is power to the switch. There is power to the light. He hoped replacing the switch would solve the problem. He now thinks it is a wiring relay problem as you say, and is trying to get a wiring diagram.