Until about about 8 months ago I was an auto detailer for several different shops. I have seen the problem several times but have never been asked to tackle it (unfortunately). I have never owned a Mustang with the lens type headlights however am farely sure (with a little cohersing) that you can remove the outer piece of the cover. Any "good" detailer will rub out plastic lenses to remove scratches and build up. By rubbing out the surface you remove a very fine layer of plastic (and any other garbage attached to it) this will bring you to a once again perfectly clear surface both inside and out. You may even try just having the outside rubbed out and see what kind of results you get. This may solve the entire problem....you never know.
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