[iainb]

I had those "extra" rear fog light sockets on my Audi, Mercedes, BMW, Saabs and Volvos - in this country (in late 60s, 70s and 80s).
I always added a bulb and connected it to the fog lights switch. Not sure, if they still have the same set up on current models (those makes).

Our 02 jetta has the slot for a rear fog bulb. We don't have front fogs so I never got around to hooking the rears up.