An abrasive polish , even something like Autoglyms SRP will do it
Cheers for that, my drivers headlight looks like it smokes 50 a day its that yellow.. I used Meguires Plast X which is designed to restore headlights etc, but it cleared it for a week, and has come back as bad if not worse.new ones cost peanuts from manufacturers like DEPO (buy for lancer 98-2000)