A rear fog lamp is a red light, about as bright as a brake lamp, 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. A rear fog lamp is very much brighter than regular taillights, and the light is usually concentrated in a tighter beam to "punch" through fog, rain or snow.
Here are some photos taken on a foggy highway:
The round lamp on the left is a rear fog.
Rear fogs may be separate or built
into one or both rear lamp clusters.
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've only just begun to show up on US-market cars over the last decade or so.
It can be tough to find a rear fog lamp that can readily be added to a vehicle with a modern flexible plastic rear bumper fascia. The export-specified Chevrolet Corvette C5 rear fog lamp is such a lamp; it makes for a very tidy installation on most any late-model vehicle. It comes as a kit, which should include everything you need with the possible exception of a socket or bulb; those are easily obtained in the aftermarket. Part number for the left lamp is 1253 3323; the right lamp is 1253 3324. Here are pictures of this fog lamp installed on a Corvette:
So, how many rear fog lamps—one or two? Regulations in most of the world, including North America, permit vehicles to be equipped with one or two rear fog lamps. If only one rear 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 the country in which the car is sold.
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 and can obscure your brake lights.
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). European regulations will soon change to require automatic switchoff so that the rear fog will need to be manually activated each and every time the driver wants to use it after shutting off the headlamps, fog lamps or ignition. This requirement will surely be slow to spread to North American regulations, if ever it does.
A good metric to use in deciding when to switch your rear fog on or off is "Do I wish the guy in front of me was displaying a rear fog so I could see him better/Do I wish the jerk in front of me would turn off that damn bright red light?".
US-market versions of European cars may or may not have wiring for the rear fog lights and some with integrated rear fogs (Mercedes W126 for example) may or may not have a socket so all you need to do is add a bulb. If you don't have a socket or wiring you will need to add them. Many cars (even including such high-end makes as Porsche and BMW) have over the years used separate rear fog units mounted below or in the rear bumper. Some cars, such as W111 Mercedes shown above, make a "pair" out of one rear fog and one reversing (backup) light. 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. Examples include late Saab 9000s, the European versions of the Chrysler Vision and Chrysler LeBaron, certain Audis, etc.