The rear fog light switch supply passes through a normally closed contact on a externally mounted relay, which is controlled by the main beam circuit. Which means that the fog light is supplied via the dipped beam circuit, but whenever the main beam is on the supply to the fog light is dropped out!!...why?
The thinking behind this was to ensure that high intensity rear lights were not left on when not required. In thick fog your main beam reflects from the fog actually reducing visibility so it follows (ishy kind of) that if you can use main beam you no longer need rear fog lights.
I have to stop typing now or this post may end in a rant about most motorsists inability to use fog lights correctly