I have a 1991 300TE 4matic with the rear fog light. The yellow pilot light in the switch indicates when the rear fog is on. My 1980 300D has the same switch but with a solid plastic insert in the middle of the switch. It doesn't have a rear fog. I'm not sure wihich MBs have the rear fog and which do not, but I'm fairly sure the pilot light works if you have the fog light.
The next porion of this thread is usually about why some European marques use one rear fog and others use both sides. I recall the Germans use only one and the Swedes use both, and I have a Volvo 760 Turbo wagon that has two rear fogs.