I just replaced my front bumper on my f150XL with one from an expedition. It has fog lights, and I found wires tucked away under there, and plugged them in. Now I just need to know how to turn them on, and off. I didn't check to see if they were working before installing the bumper.

 

If you have the plugs under the bumper you are probably fully wired under the hood. But I wouldn't be surprised if you are missing the wires from the firewall back for the switch. There are a few threads around for adding fog lights if you search deep enough.

If you are missing the wiring from the firewall back you need to find a safe way to add a switch to energize the coil of the fog light relay.

 

You didn't say what year your F150 is, but the fog light switch on my 2001 XLT is the light switch, you just turn your lights on and pull on the light switch.