Fuse 15 is for fogs, IIRC. DO you know if the fog light rewire has been done?
Were it me, after checking the fuse, I would pop the fog light switch out (via the tab on the side of the switch) and probe for juice to and from the switch.
If no juice to the switch, I would rewire the lights with a relay (as opposed to doing this later or separately, as it is a real good idea anyhow). If you have juice leaving the switch, I would trace the wire for an open. Also make sure you have grounds at the bulbs. I doubt your issue is past the switch though.
Most of the time, people will find a new source for the fog lights while doing the rewire (so that one can have fogs on with parking lights, etc). So a new wire at the back of the fog light switch connector is a tell. This is one part of the rewire (new source and the relay are both parts. One can do one without doing the other though. The relay is most important IMHO).
There are various write ups that people follow for adding a relay (the Corral has a popular one, which does the relay and a new source). IIRC, on that write up, they have you put the relay down behind where the switches sit. So if you see a relay back there, probe away and see if its N.O. terminal goes to the fog lights.
Mine is in the engine compartment, which is easy to see (I have lows, highs and fogs on relays). Or if you hear a click when you turn on the fog lights, that indicates a relay is in there somewhere. I would hunt for the click to find it and test the terminals.
If your headlights ever flicker out after driving for a bit (with headlights and fogs on), that suggests you dont have the relay on the fogs (that is a reason people do the rewire). The headlight switch has thermal overload protection, so it will shut off all lights till it cools down (like many hairdryers).
This should be no biggie to fix, esp if you want to add the relay and a new switching source, since doing that will likely bypass your issue.