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 vehicle without a rear fog is closer to the camera, but the vehicle with the rear fog illuminated is easier to see.

Two vehicles with rear fogs illuminated.

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. Research has shown it's safest and best to have a pair of rear fogs, because that way they provide observers (the people behind you on the road, whom you don't want to hit you) with not only a cue to your presence but also the distance between the observer and your car. It works the same way as getting a sense of how close the car in front of you is by the apparent separation between the two taillights or brake lights—the separation appears to grow wider as you approach the car.

A single rear fog lamp is better than none, and is allowed in deference to automakers' desire to keep costs down and maintain maximum design flexibility. Some vehicles come from the factory with rear fog optics and sockets on both sides, but with only one wired up. It's done this way because the automaker wants to provide only a single rear fog lamp. ECE Regulations and SAE guidelines state that if only one rear fog is fitted, it must be located between the vehicle's centreline and the outermost extent toward the centre-of-the-road side of the vehicle, and which one that is depends on whether the car is to be operated in a left- or a right-traffic country.

A pair of rear fog lights is really the better way to go in terms of minimising the likelihood of getting hit (which is the primary main objective), but it's important to position them with good separation from the brake lights, so the rear fogs don't mask the brake lights—these two kinds of light look very similar, so if the rear fog lamp is too close to the brake light, it is hard for following drivers to see when you apply the brakes. ECE Regulations specify at least 10 cm (~4 inches) between the closest lit edges of a brake light and a rear fog light. Here again: better than nothing, but the research shows this separation should be at least 13 cm (~5 inches).

Many people don't know or care enough to use their rear fogs properly. It should not be turned on just because it's raining or snowing or dark out, or because you want to show everyone your car has rear fog lights, or for any reason other than you wish the driver in front of you had their rear fogs on because you're having trouble seeing their car through the rain, fog or snow. Rear fogs are like front fogs in that you shouldn't have them switched on unless it's genuinely soupy outside. (linked article opens in new window). It's also very important to remember to turn OFF your rear fog lights as soon as they're no longer needed. Misuse of rear fog jlamps causes glare to following drivers and can obscure your brake lights. Here again, when conditions are such that you'd object to a glaring, bright red light in your face...make sure your rear fogs are switched off.

Rear fogs must be wired up correctly. It's not safe to camp them onto the front fogs so the fronts and rears come on together, and a simple on/off switch isn't adequate, either. What's needed is a setup which allows the rear fogs to be switched on only when the headlamps or front fog lamps are active, will automatically shut off the rear fogs when the headlamps or front fogs or ignition switches off, and will require the driver to deliberately switch the rear fogs back on again after that. Also, there has to be a telltale—a pilot light visible to the driver—lit when the rear fogs are on.

Such a switching setup can be put together from scratch with some cleverness and ingenuity, but it's easier to pick up this appropriately-configured switch-and-relay kit. The relevant parts of the description translate to English this way:

Rear fog light package/aftermarket automatic fog light (16mm button) (Fog lamp itself is purchased separately). Choose black [svart] or stainless [rf/silver] button. A rear fog lamp is not included in this package. This kit has the devices you need to get a complete and approved rear fog light function in the car. The package contains a relay, momentary/spring-loaded fog light button-switch, a socket for the switch, and one for the relay. It also comes with adhesive-lined shrink tubing. You supply your own wire and rear fog lamp(s).

Pressing the switch turns the rear fog light(s) on or off. When on, the rear-fog logo on the button lights up in yellow-amber. If the power to the relay is interrupted (e.g., the ignition is switched off) while the light is on, that switches off the lamp and it stays off even if power is restored to the relay; the button must be pressed again to turn the rear fog lights back on. The linked outfit offers rear fog lamp units, but you can spend less money and get better lights; send me an email and I'll be happy to advise.

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.