You could take one of the same relay from another slot and try it to see if it works.

Edit - My fog lights currently don't work either, though I'm not even sure how to turn them on, I just use my brights when needed. (Took the working relay from my fog lights to make my horn work properly).

 

when i first got my truck the horn didnt work BUT the fog lights did than magically one day i realized i had no fog lights dont know if they whent at seperate times or what... in my truck theres a *** to the left of the steering wheel if you pull it half way its fog lights all the way is both fog and head lights (when they were working)

 

Yea then my fog lights never worked. I've done that and gotten nothing from doing so, but you could try and replacing one. Like my old taillight issue, replacing one can make the other work because they hit the same relay and cause the other to stop working. so you could try switch the bulbs around, maybe clean the sockets too.