What happens is the plastic itself degrades and turns yellow and clouds up.That's why the lens restore kits only last a few months before the problem is back.
thats the oxidation there... reason it turns yellow is cuz just like the 3rd tails on the 99-04's stangs theyre left unprotected... so the elements are literally eating the plastic... some guys with s197's have resorted to clear coating their giant fogs before they have a chance to start... i did that with my 3rd tail light and hasnt come back... pondered doing it to my head lights but kinda weiry of doing that on headlights...
but i agree.. it will come back... never said it was a perminate fix.. even the replacement units suffer the same problems for the sn stangs... couple years and theyre ready for the dumpster again...