I've used Brasso metal polish and polishing compound on the Mustangs lights, either one works well, but the oxidization comes back of course.
I can say first hand that clearcoating the lights works, I did it on my old Escort. I sprayed carb cleaner on the headlights, it melts the layer of plastic with the oxidation on it, then I sprayed clear on the headlights while the car and they never yellowed again. I found the controversial carb cleaner method on the Escort fourms, but dont do it on a car you care about. lol