I'm kind of disappointed. Several replies and much incorrect information.
First, look around your Mustang's interior. See all of the plastic? Know what it's made of? That's right, they're petroleum products. Vehicle interior plastics, ESPECIALLY when the car is new, give off oily fumes that leave an oily film on the inside of your windows, which must be removed by cleaning your windows pretty often. Those fumes and the film they leave on your windows will cause them to get foggy fast, as soon as s little bit of moisture gets into the cabin.
So, wash the inside of your windows often, especially in the first year.
Next, recirculation is defintiely how you want your AC set when you have fogging windows. When you turn the windhield defogger on, two things happen. First, the AC turns on, and then it goes into recirculating mode. Yes, that's correct, when you turn on your windshield defroster/defogger, it automatically goes into AC/recirculate modes. The dash lights may not light-up to show this, but that's exactly what it does. So, when trying to defog ALL the windows inside the car, (like when you have multiple people with you, giving off more moisture) use the AC in recircualting mode. The AC keeps recirculating the same atmosphere, removing more and more of the moisture in it, which will help your windows to fog less, and making the air inside the car more and more dry.
Put it another way. Do you know what AC does in addition to cooling atmosphere? They're de-humidifiers, and the more you recirculate the same atmosphere, the dryer it gets.
So, keep your windows clear, and have your AC set on recirculate, espcially when it's humid outside. If you don't, your AC system is only getting one pass at removing the moisture inside your car, instead of continuing to remove more and more of it.