Hm, you're right about the icon but I don't know if I've ever seen a 180SX rear fog. Only on 200SXs with the larger rear bumper cutout. There's also none in the parts diagrams. I wonder if that's a 200SX switch that made its way to Japan, but the 200SXs I've seen had a defrost button there (and a headlight leveling control in place of the coin slot).
I was thinking of
this kouki 180SX fog light kit but that seems to have used a double throw switch labeled I/II.