You can operate your rear fog light exactly like JCE said. If your car is a 124 and the fog light was mentioned in your manual then it is quite likely that you have it.
My old 1981 300SD had the second detent on the light switch for the rear fog, but it did not have the light. The 1983 280SEL has the rear fog light, but it is a European edition and the rear fog light is required by law in Germany. My 1991 300D has the rear fog. Perhaps Mercedes eventually got lazy and just decided to include the light on every car, including those intended for the US.
1976 300D (W115) - 330K and still going (sort of)
1991 300D 2.5 Turbo - Sold at 221K
1983 280SEL - Sold at 206K
1981 300SD - Sold at 232K