I've had some pretty deep scratches in my plastic
headlights and the only way that worked for me was wetsanding with 3M Imperial Wetordry 400-grit, followed by 600, 1000, 1600, 2000. I then hand polished with
Meguiar's #17. The trick here is to really work the #17 until it gets tacky, and repeat several times until it's crystal clear and squeeks with each pass (I use 100% cotton towels). I then topped off with
Meguiar's #10. The whole process could take a few hours, but it's worth it - my lights look practically brand new whereas they were yellow and pitted before.