Rear fog lamps (or "fog taillamps") have since the 1970s been mandatory equipment on all cars sold in countries that adhere to European (ECE) vehicle safety Regulation 48, which governs the installation and hookup of lights and reflectors. But, they're only just beginning to show up on US-market cars. They are intense red lights manually switched on by the driver in conditions of very poor visibility (heavy fog, rain, snow) to increase your car's visibility to vehicles behind you. A rear fog lamp is very much brighter than regular taillights, and is in many cases even more intense than brake lamps. Regulations in most of the world, including North America, permit vehicles to be equipped with one or two rear fog lamps. If only one rar fog is fitted, it must be on the driver's side of the vehicle. It's safest and best to have just one rear fog, much as it offends the sense of symmetry of those who don't know any better. Having two is legal, but makes it much more difficult for following drivers to see a change in your car's rear lights when you step on the brake. Many vehicles that do come from the factory with rear fog functionality have rear fog optics and sockets on both sides, with only one wired up -- this isn't a factory oversight. It's done this way because the "driver's side" of the car varies depending on which country the car is sold in!

US cars may be retrofitted with them; the headlight switch on virtually all Mercedes pulls out two stops - the first lights up the front fogs, the second lights up the rear fog. Many people don't know or care enough to use their rear fog properly. It should not be turned on just because it's raining or snowing, just because it's dark out, just because you want to show everyone how cool you are for having a rear fog, or for any reason other than your wishing the guy in front of you had one turned on because you're having trouble seeing him through the rain, fog or snow. It's also very important to remember to turn OFF your rear fog as soon as it's no longer needed — misuse of rear fog lamps causes glare to following drivers. Some vehicles' switching arrangements are poorly thought-out, making it possible to leave the rear fog switch on so the rear fog comes on whenever the front fogs are switched on. This is improper and unsafe, and besides, you shouldn't have your front fogs on unless it's genuinely soupy outside! (linked article opens in new window).

US cars may or may not have wiring for the rear fog lights and some with integrated rear fogs (W126 for example) may or may not have a socket so all you need to do is add a bulb. Do not use an overwattage bulb; the correct item is a P21W. If you don't have a socket or wiring you will need to add them. Cars such as the W108 and W111.026 (shown above) may have separate rear fogs. The the car pictured above the red light is the rear fog light, the clear light is a reversing light. They are wired separately, not as a pair. This is also a fairly common arrangement where the rear fog is integrated into the taillamp clusters -- the driver's side cluster has a rear fog, and the passenger side cluster has a reversing lamp. (In cases like this, the single reversing lamp often doesn't have enough oomph to safely light your way backwards. There is a safe and effective upgrade bulb, number P796, available from Candlepower.)

Here are some photos taken on a foggy highway: