The plastic headlight covers on my vehicles were hazy, faded, and yellow. The original factory clearcoat was damaged by the weather and UV sunlight. Instead of attempting a temporary fix, done by many with toothpaste or some kind of polish, I permanently restored my headlights. I first wet sanded the headlights with increasingly finer sandpaper. After cleaning, I applied a 2K clearcoat from a rattle can (see Amazon link below). The headlights on my Lexus RX350, BMW 128i, and GMC Sonoma are now beautifully transparent and shiny. The new clearcoat will protect the plastic from UV damage and last for many years. The quick 20 minute job was well worth the effort.
2K clearcoat on Amazon:
https://amzn.to/3OXNnVD
Automotive sandpaper assortment on Amazon:
https://amzn.to/3OYDYNA
Isopropyl alcohol for cleaning on Amazon:
https://amzn.to/49rUePj
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Part of the reason for dim, yellow headlights is that the halogen gas in the bulbs has been used up.