Especially when weather conditions are variable, as in winter, you may find yourself following a car or truck with a bright red light that is always illuminated, regardless of what its brake lights are doing.  Those little red lights are the rear fog lamps. If you?re an American, you?ll be forgiven for not knowing such a light exists on many cars and trucks. Not everyone is privy to UN Regulation 48, which spells out lighting and lighting-signaling equipment standards to which vehicles sold in European countries must adhere.
The regulation is spelled out in a 128-page document by the United Nations Economic Commission for Europe. About 10 pages of it are dedicated to prattling off definitions for words such as ?lamp,? ?light source,? and ?ground.?** There are also guidelines for the intensity and function of rear fog lamps, which are required in European countries. In general, a vehicle can be equipped with two or just one of the rear lights, but if there is just one, it must be on the driver?s side. We checked some of the European vehicles in our fleet and, sure enough, they had a little rear fog lamp switch or button, not far from where you find the toggle for the front fog lights. Here’s what that switch looks like on a 2016 Range Rover Td6 diesel:

And here’s what it does:

See that bright red light at the bottom of the taillamp"
Here’s the setup on a 2017 Audi A6 2.0T, which has a button on the bottom left:

And here is what it looks like ...