I've wrestled with headlamp restoration for years on my Mazda. The crucial thing is sealing the headlamp after polishing. Back in the day the only sealer they had was this product called blue magic(could've been genie?) and it sucked. I've experimented with many types of waxes, from Merzerna Power lock to Black Magic Full synthetic(discontinued but it was great stuff)... I feel like Merzerna had a hard time adhering onto the polished plastic because it only lasted about a month.
For the past 2 years I've used deck sealer.. helmsman polyurethane... it lasts about a year then begins to flake and peel... I think this time around I'm going to just keep applying Turtle wax seal n' shine every month so I don't have to keep sanding it down once a year. I've seen some others use nano ceramic sealant, but the good stuff costs about $75... which is more than I'm willing to pay for sealant right now.