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Bought a used headlight a few years ago. It did not have back covers. I figured no need for them.
Now I regret not buying back covers then. Yes, the inside got kind of yellow because dust and humididty got inside. There is a three year old dead insect in there too.
Do yours have back covers or have they been modified in any way that there may be an opening letting mosture/dust in them? That may be the reason for the yellowing inside of headlight.
Old thread... but, if the yellowing is inside the lens and not outside, you'll have to open the headlights up and do a refinish (would also highly advise a UV-resistant clear coat applied after refinishing). Might be better off just buying a new set of lights though.