There's a few ways to go about it.. The most popular/used way is to buy a Headlight Restoration Kit pretty much everywhere but Autozone, O'reillys, and Wal-Mart have the best selections. You could also get some toothpaste ( Ultra Brite, or any Whitening type paste ) and put some muscle into it and you may have to do it for 5-10 minutes each side and then leave it one there do the same process again, but this time when you are done wipe it off, see how it looks and if not satisfied do it once more and you should be good!.. It really just depends on how severe the discoloration/staining is. One more way is " Wet Sand Resto " and it's a 3 or 4 step process where you use sand paper in different abrasiveness and start with the most abrasive and go down to the lightest. My personal preference is to use one of the restore kits. Makes the process a whole lot easier and you get really good results every time! . FYI i use either Quixxx kit or Sylvania's kit and they can be found at basically anywhere.
Just a quick thought have you thought about just picking up a set of Headlights ?.. You can pick up a pair of aftermarket headlights online ( ebay, mustang specific sites online ) and you can get a set that is smoke tinted dark, crystal clear, or all uniform with no orange/amber for the turn signal part of the headlight for between $40-$60.. just thought bro. Let us know what you end up doing! Take care!!
Thanks to both of you who responded. This is the first car that I have had this problem with. I think I'll try a headlight restoration kit first and if all else fails buy the replacement lights. It may be a week or two before I get to this but I will report back.
I tried a couple of different restoration kits & they both looked good for a while but in reality it was just putting lipstick on a Pig. The lenses always fogged up again within a few weeks or months b/c the protective coat that was originally on the headlights is oxidized or gone completely. I broke down & bought a really nice set of dark smoked headlights with Xenon bulbs already installed for about $150 on Amazon.com. It was honestly one of the best bang for the buck mods I've made to the exterior of the car, they look fantastic.
O.K. so it turns out that with the fogged headlights the car will not pass the Ontario safety inspection which I need to transfer the ownership. Since I had it in the shop for the safety anyways I decided to have them install a new set of aftermarket headlights which I found and bought separately. I did keep the old lights so that at some future date I may experiment with a restoration kit. For now, I am good to go.