Rear Fog Light Switch?

Hi welcome to the forum
Your car will require a form of "tell tale" to indicate when the rear fog lamp is switched on , this may well be by a small tag on the switch itself that lights up, it may just be the small bulb within the switch has blown and easy enough to replace.
You can make your own if you want as long as its secure and functions ok.
Out of curiosity what vehicle is it and we may be able to tell you where the tell tale is?