Jake850,
As Ray has already mentioned, it is a standard that manuafacurers are using as not to be confused with brake lamps. I am seeing more and more rear fog lamps on late modeled luxury cars lately. Always the same left side. What I did, (just to be different and set my 850 apart from others, I installed a lamp on the other side (breaking the protective "do not touch" tab first and installing a pressure spring as in the other side). I was carefull to pick a wattage that is brighter then the tail lamps by just a hair, but not as bright as the brake lamps. On very rare occasions when the weather is really nasty with winter storm or whatever, I turn them on, giving my tail end more "red" to drivers following behind.