So detailing here in Southern California where we have more than abundant amounts of sunshine, lol, I get a ton of clients cars with yellowed headlights. I've tried every kit there is to buy that claims to restore headlights. They all suck. I always stick with what my mentor showed me for headlights. Several runs of a wet sand, as
@aces-n-eights mentioned. Sand a couple passes with 600-1200 grit sandpaper. However my method is different after the sanding. I take my rotary polisher with a compound I buy from my supplier from their V-Line polish, usually the heaviest which is called V32. That on a hex logic polishing pad, yellow, and it takes the haze off. Usually a couple passes although I have to be gentle as the rotary polisher will do a lot of damage very quickly if not used correctly. Once the lens is cleared of oxidation, I finish with several coats of a specialized 6 month paint sealant I buy from my supplier called jet seal. I usually use jet seal as a winter time paint sealant for my customers here where I live in the mountains as it is designed specifically to protect paint through the cold and snow we get here. It works phenomenally. Finishing the headlights with several at least two coats of jet seal will prevent headlights from oxidizing again. Regular wax can be used to seal the lens but it will ware off ad need to be reapplied often in order to prevent headlights from reoxidizing. I just did a set of headlights on a 2005 Denali xl that a client has for sale before the weekend. They come out pretty nice. I'll grab the pics from my phone. Although I couldn't get the running light lenses to clear. The angle doesn't allow for enough surface area of the polishing pad to make contact with the lens to polish. So I'm looking for a solution to that problem.