just a thought, as i spent a little time yesterday polishing my headlight lenses only yesterday.
i have some slightly discolored areas around the top area where the "horn" is, what i found is that there was some minor pitting from road debris (probably sand) and that pitting was /is collecting dirt , that seems to be causing the stains.. basically, their getting sandblasted..
I used some plastic polish(from .Griots Garage ) and a soft cloth cloth and it helped. it takes some elbow grease, and i have to work on it more. i followed that up with some plastic cleaner(bought from griots as well) and It definately made a difference and the pits/scratches in the area i did are far less visible now.
Im wondering if that 3m type super clear shield like this can be made to fit our headlights, but i havent checked into it yet. Once i get them completely polished , il call around and see what the vendors say...
but maybe this will help..