they are optional to turn on... there is no automatic process to it....
OK, in almost all the European countries there are areas on the auto bahns where the fog is very intense and thick, and believe it or not the drivers will not even slow down for the fog. They will continue to run 100 plus mph or 160 kilometers and just turn on the front and fear fog lights so that other drivers will see them coming and going and the slower drivers do the same so the faster drivers will see the slower drivers. These rear fog lights are super bright red lights (can be confused as a brake light and formula-1 cars use them too) and most people are unaware that they even have them on their cars in the states. Volvos, VW, Audi, Porsche, Mercedes, BMW and some other European car companies put them on their cars standard and the American consumer has no idea what they are for or do. Just another button to push or turn depending on where the switch is. So you will see people driving down the road with this really bright red light coming from the back of their car and they have no idea, they think they have turned on the front fog lights... really pisses me off because people buy these cars and have no idea what rear fog lights are and what they are used for, seriously. Some cars have the rear fog light just below the brake light on the driver's side like Volvos and some cars have the rear fog lights on both sides just inside the brake lights like BMWs... I use to get people off my tail that drove to close to me by turning my rear fog lights on and hitting the gas. The effect was the guy behind me slowed down because he thought I had hit my brakes and I would pull away at a faster speed, would really piss the guy off behind me, but I hate drivers that ride to close to my bumper at high rates of speeds. Anyway I hope all this helps. They really are a great asset to have if you live in a place with bad weather or fog for most of the year, like Europe and if so then they work and serve a purpose... man that was a long one...
enjoy