For all three vehicles that I own; about 4, 8 and 17 years old, same light problem.
The headlight clear plastic fade/frost over and/or develop spider crack lines requiring sanding.
Yet, the side marker and turn signal clear plastic (of the same level of clearness) look brand new. The red tail light plastic all look brand new.
So why is it the headlight plastic on cars fade/frost/spider crack but the plastic used for all the other exterior lights never ages?
Headlight's light source is bright and generates a lot of heat, and that's behind the plastic headlight lens. They're subject to all the road conditions, weather, dirt/dust, thermo-cycles etc. getting all the contaminants thrown at it every day. The dirt and dust is like driving thru very fine sandpaper and being softer than glass that stuff will embed damage the headlight lenses. The other lights emit lower energy (watts) and used less frequently, and tail light lenses are colored red, so would be difficult to see anyways, but even then the sun's uv will cause the tail lights to fade eventually if parked outside all the time.
It's really atrocious. I much prefer glass headlight lenses. (My daily driver has them and they're still perfectly clear after nearly 25 years.) Manufacturers prefer plastic though because it's cheaper, and it's easier to form into complex shapes. So what if it turns opaque after a few years, they want you to buy a new car by that time. Just sign and drive - and pay, pay pay..
https://www.youtube.com/watch?v=NSqoL2S8WPI
Maybe because the headlights use bright lights compared to the others? Or maybe because the headlights are at the front of the car, so they go through the most stress and get hit by a bunch of debris? These are just guesses lol, but good question you bring up.
Probably because it is not same material. Heat from bulbs probably also contributes to fading.
I always thought it was because of all the tiny road debris hitting it over the years.
Look at an older cars windshield (it it hasn’t been replaced). It’s not as smooth as it used to be. Same idea with the head lights.
