Not sure if the issue of rear fog lights has been covered on the forum before -but would be interested in seeing what others have done or experienced.
I spend quite a few weeks a year driving in the alps mainly for skiing. When I purchased my xc90 I checked to see how many rear fog lights it had and the answer was one on the offside. This is perfectly legal in the UK as a single fog light must be either offside or centre. However the same rule applies in Europe and that therefore means that if I took my xc90 to the alps with just the one foglight it would be on the 'nearside' and therefore illegal when used in fog!
I pointed this out to my dealer who said they had never had this raised before - which I was very surprised at! Anyway before buying the car, volvo wired in the second foglight (FOC) so it is legal here and in Europe.