I am not sure on your car but on my 1988, 190e, the rear fog relay is incorporated with the intermitant wiper relay. It is a large black box at the back within the fuse box under the hood on the driver's side.
I do not know if your car is wired the same but by chance are your intermitant wipers not working.
To fix mine, I took out the relay cleaned the conenctors and reinstalled. No problem since!
Insanity: doing the same thing over and over again and expecting different results.
2006 - Suzuki Gran Vitara (2.0 L fully equipped) Like this car so far except for trying to put on the seatbelt.
1988 - 190e - 2.3L - 172K miles (It now belongs to the exwife)
1999 - Chevy Blazer LS Fully Equiped - killed it June 2006
2001 - Honda Civic EX - 68K miles (sold June 2004)
1963 - 220S - Dual Carb 6 cyl. (sold)
1994 - Yamaha WaveRaider (fun to ride)