What is a "rear fog light"?
A rear fog light is like a running light/parking light i.e. red, except markedly brighter. It serves to make a car more visible to traffic behind it in inclement weather. In Europe, the rear fog light is on the driver's side, if there is only one. Cars in the U.S. generally do not have rear fog light. There is a distinct control switch for the rear fog light i.e. there is a separate switch, or a separate position on an existing switch, that turns only the rear fog light on or off.
As with the front fog lights, the rear fog light should not be used casually, for instance, when the weather is clear. Since the rear fog light is quite bright, sometimes as much as 35 watts, when it's on, it can obscure the brake light and turn signal. (The regular parking/running light's bulb is usually 5-8 watts.) The brightness is also very annoying to traffic behind the car.
Does the U.S. MkV GTI have rear fog light?
The U.S. MkV GTI does not have rear fog by default, so it cannot be simply "enabled" using VAG-COM. It is possible to turn the driver's side back-up light into a rear fog light. You will need to swap your OEM U.S. headlight switch for an Euro switch, add a "trigger wire" (VW Part 000 979 009) between the Euro headlight switch and a connector under the dash, move an ECU wire from the rear back-up light position to the rear fog position, and do some VAG-COM'ing. (You will need to replace the driver's side back-up light's bulb with a red bulb.) Here is a PDF (warning: the PDF is 12 MB!) with the full details.
What about Euro LED tail lights for the U.S. MkV GTI?
"GTI Euro Tails" [vwvortex]