I have a 04 Prius and the HID headlight housings on both sides are fogging over like cataracts. Is this bad design with a costly replacement?
The clear plastic housing ages with exposure to sunlight. Try using plastic polish on the exterior to see if that helps.
I've had good luck with Meguiar's plastic cleaner and plastic polish. Use the white bottle first, followed by the blue bottle. For extreme cases, there are kits using fine sandpaper, which is followed up by polishing compounds. You can search youtube for some impressive demonstrations on pretty degraded headlight lenses. After they've degraded once, it seems the plastic is more susceptible to future clouding, but that doesn't mean you can't clear it up again with a little elbow grease!
Always mask around the lights to protect the paint before attempting any headlight polishing operation.
All depends on what you mean by fogging over. If there is a crack or hole in the plastic lens, moisture gets in. Small cracks or holes may be repaired, otherwise replacing the housing is in order For fine scratches that resemble hazing, that can be buffed out with the appropriate plastic cleaner. All plastic headlight lenses will become hazed/scratched/yellowed with age I've had very good luck with this product Sporty's Windshield Care Kit - Sporty's Pilot Shop You can also get automotive-style products, like this one from Eastwood Clear Plastic and Headlight Restoration Kit Hope this helps
Here you go. http://www.spadepot.com/shop/Novus-Spa-Scratch-Removal-Kit-P757C57.aspx I've used it on the headlights on my 2000 Camaro and cleared them right up. The plastic polishing compounds made by Meguires (Plastx) and Mother's (Plastic Polish) are excellent polishes as well, but to get off the road grime on the plastic you need to use a progression of abrasive compounds in order to get off the grime and remove the abrasions made by the previous compound. That is all included in the Novus 3 step scratch removal kit.