The rear fog light isn't like the front fog light. The rear fog light is basically the brake lights at twice (or so) of the brightness. Easier to be seen in the fog. You can usually see some here in the U.S. -- ever see a MB and think, "Wow, that dude's brake lights are bright". He has the rear fogs on and doesn't realize it.

 

The regulation here says that the rear fog lights are only allowed to be switched on when sight drops below 50 m, and max. speed must be 50 km/h. -I have no idea why the speed limit, because it would make sense to me to use the lights on the highway at higher speeds-. But it's right that many people don't switch them off when they have someone following. It's pretty blinding to drive behind a car with rear fogs on!