Best way is to rub down the outer plastic with increasing grades of wet and dry. I did it recently...
Tickled with the front lights today:
Was full of little scratches and pits from 13 years on the road:
After some 400, 800, 2000 grit rubbing, compounding and polishing its not perfect, but much better:
Rough:
Mid - lubed up:
Finished:
You can see from the before and after light pattern on the outer plastic that more light is going through rather than being scattered by the pits.
Then fit some decent bulbs such as Halfords Extreme brilliance etc (when on BOGOF) and that'll be about as good as you can get it.