Hi Zak. I fitted a couple of round Hella fog lights to my 280SL, mainly for looks. Not to everyone's taste no doubt, but it's a personal choice. I found that the fog light position on the switch was fully functional, so wired them accordingly.
Incidentally, also found that the left and right parking light function worked. The tail lights already had the diodes in them. Not really of much use, but a throwback to European requirements, and fun to demonstrate. There is an excellent color and connection diagram of the h/light switch in the forum technical section. Cheers, Dale.