The Rear foglight can be either on the center of the vehicle, or on the Driver’s side. You can also mount them in pairs.
Rear fog lamps
In Europe and other countries adhering to UN Regulation 48, vehicles must be equipped with one or two bright red “rear fog lamps”, which serve as high-intensity rear position lamps to be turned on by the driver in conditions of poor visibility to make the vehicle more visible from the rear. The allowable range of intensity for a rear fog lamp is 150 to 300 candela, which is within the range of a US stop lamp (brake light). Rear fog lamps are not required equipment in the US, but they are permitted and are found almost exclusively on European-brand vehicles in North America. Audi, Jaguar, Mercedes, MINI, Land Rover, Porsche, Saab, and Volvo provide functional rear fog lights on their North American models. Some vehicles from non-European brands which are adaptions of European-market offerings.
Most jurisdictions permit rear fog lamps to be installed either singly or in pairs. If a single rear fog is fitted, most jurisdictions require it to be located at or to the driver’s side of the vehicle’s centreline—whichever side is the prevailing driver’s side in the country in which the vehicle is registered. This is to maximize the sight line of following drivers to the rear fog lamp. In many cases, a single reversing lamp is mounted on the passenger side of the vehicle, positionally symmetrical with the rear fog. If two rear fog lamps are fitted, they must be symmetrical with respect to the vehicle’s centreline.
Proponents of twin rear fog lamps say two lamps provide vehicle distance information not available from a single lamp. Proponents of the single rear fog lamp say dual rear fog lamps closely mimic the appearance of illuminated stop lamps (which are mandatorily installed in pairs), reducing the conspicuity of the stop lamps’ message when the rear fogs are activated. To provide some safeguard against rear fog lamps being confused with stop lamps, UN Regulation 48 requires a separation of at least 10 cm between the closest illuminated edges of any stop lamp and any rear fog lamp.
Taken from wikipedia’s automotive lighting article.