[Lil bastard]

Rear Fog lights are mandated in Europe and any countries following ECE regulations (ECE Regulation 38, SAE J1319).

But in these countries, the use of Rear Fog lights is only permissible (and mandatory) at visibility conditions below 50 meters (151 Ft.).

This is because the luminosity of the Rear Fog lights is thirty times more intense than that of the Rear Running lights (Europe) and could interfere with the vision of following drivers.

Rear Fog Lights must also be capable of operation independent of other running lights.

Either a single or double Rear Fog lights is permissable, but if a single Rear Fog light is to be used, it must be on the Driver side of the vehicle's centerline — whichever side would be the driver's side in the country in which the vehicle is registered. This maximizes the sight line of following drivers to the rear fog lamp.

If dual Rear Fog lights are to be used, they must be symetrical in relation to the car's centerline to prevent them being mistaken as Brake or Running lights by following drivers. To further provide some safeguard against Rear Fog lights masking as Brake lights,, ECE R48 (R=revised) requires a separation of at least 10 cm between the closest illuminated edges of any Brake light and any Rear Fog light.

There is no regulation requiring the use of Rear Fog lights in North America.

But, many European manufacturers include them anyway and call them a Safety Enhancement when in reality, it is cheaper for them to produce one standard Tail Light, than market specific ones.

Since the EU allowable range of intensity for a Rear Fog light is 150 to 300 Candelas, which is within the range of U.S. Brake light regulations, Rear Fog lights are more easily confused with Brake lights here than in other countries.

Porsche includes Rear Fog lights as a Safety Enhancement to North American Cars. As in EU countries, Porsche uses only a single Rear Fog light (for safety sake with respect to the following drivers vision), not dual lights as has often been speculated here. They have a socket for the lamp in both the right and left Tail lights, again to eliminate producing market specific Tail Lights. And since they install inversed wiring harnesses, depending upon whether the car's intended market is left or righthand drive (which side the Steering Wheel will be on), wiring is only existent consistent to that market.