What year is your 'Stang?? If it is under warranty, get them replaced. I have had the
headlights replaced on my 01 GT 3 times under warranty. I just ordered a new set after one of them got hit by a rock but, they were fogging. They also have the fingerprints inside both lenses, it happens during manufacturing.
Here is an article on how to do clear corners on 99+ Mustang
headlights. You may not want to do that but, it does go over in detail on how to take the lenses apart and put them back together.
http://www.fnsweet.com/garage/clears.shtml
You can get replacement housings for about $90 a piece so you have to decide if it is worth the trouble to take them apart and fix them or just fork up the bucks and replace them.
Even when each set of lights were new and crystal clear they were NOT bright. It is a common complaint. If you want them brighter you might want to invest in some Sylvania Silverstars. I wasn't that happy with them and am back to stock but, opinions vary
