For those that have a 2011 - 2012 V6 Mustang
, you'll need both the grill and front bumper cover off of a GT to do this conversion.
The 2013 to 2014 V6 and GT's shared the same grill and lower front bumper, so just a grill swap is needed for those years. And don't forget the light switch. If your Mustang did not come with lower fogs (not sure if all 2013's and up got fog lights as standard equipment), you may not have the light switch to operate them.
Starkey Offers an excellent wiring kit for the 2011 to 2012 Mustangs that want to do this, and the kit includes the factory light switch replacement; as a "bonus" of sorts, that factory light switch also netted V6 owners without fog lights initially the "auto" headlight feature too (all Mustangs in 2011 and 2012 have the photo sensor on the dash for this; not sure on the 2013 and 2014's).
Here's a link for your 2013 for the Boss style fog light kit (It's pricey, but an excellent kit that has it all):