Re: Rear fog light?
My 2003 A6 2.7T MT6 has the rear foglights. The first detente on the switch turns on the front fogs and the second turns on the rears. This is a pair of red lights on each side of the license plate, not in the rear tail lights. And I use them in blizzard conditions and heavy rain only, so idiots in 4x4s don't run into me (they think 4wd = 4w stop). I also flash them when someone tailgates me so I don't have to brake, and they are bright enough to freak out and back off most tailgaters (other than the hard core jackasses and those that are busy texting or talking and not paying attention to the fact that they are 20' behind me at 75 mph). It's doubtful you can retrofit them to the XF for a reasonable price.