wal mart sells a good plastic polish for a couple of bucks. I use a dremel with one of those white polishing wheels. Use low speed spread some of the plastic compound on the lenses, use a small spray bottle with water to keep it wet and take your time. I do all my cars monthly, they look new.
this sounds crazy but it will work as long as they aint cracked but toothpaste on a toothbrush and brush them just like you would your teeth.this will help them alot...you can also get some buffing compound and that works faster and really better.
I used a paint buffer, and just buffed them, they shined up way nice
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2000 4.2L V6 Power! Mods done: Pulleys, CAI, Cams, Pistons, Bored out, Stroker kit, BBK Hi-Flo CAI with MotorVater spacer and 65mm Throttle body, Shorty Headers, True Dual exhaust with H Pipe and Cherrybomb Pro mufflers. 4.10 Gears, Ford Racing Computer, Performance spark management, Short Throw, HID Headlights, clear corners, Cobra body kit all around (except hood, spoiler, emblems and rear bumper.)
Future Mods: Cobra hood, Black wheels, Cobra R or Saleen Wing.
A friend that used to own a body shop refinished mine; I think the process involved buffing and polishing the headlights, he also removed the alignment points on the front surfaces (makes washing and wiping the front of the car so much easier without those points sticking out to rip my microfiber cloths or my hands). I had previously purchased a set of after-market headlights, but they were definitely inferior to the original headlights on the car.
mine were bad. one of my 1st pics of my car had them really cloudy. i just wet sanded them. i had 1500 grit and worked my way up 500 grit at a time till i got to 2500. i never got the time to hook up the air compresser and buff them really nice. here's the pics. before and after.
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35th Anni. Laser Red 3.8 coupe
Present- (sleeping) Repainted, Xenon scoops, graphics.Gt cut offs with H-pipe, 35th Edition Gt wheels. A chevy Blazer is getting me through winter Future- t-lock and tuner, possiable 4:10 gears.
How to clean cloudy lights 
