I've just realised that the S3's only have a single working rear fog light.
Personally i prefer the symmetry of both of them working. I've taken the rear of the clusters apart and moved the left hand side to the other side and connected it up with a bulb fitted and it works. It must therefore be a limitation in the wiring loom.
I'm thinking of connecting it to the right hand side light so that they both work. Anyone already done this that can give any advise?