I dunno... unless something might be 'growing' on the inside of the glass, I'm pretty sure it shouldn't turn yellow. Maybe they had something over the lens' to give the light at night a yellow tint?
Ever see older Neon headlights? Cheap plastic that 'cataracts' so badly you can barely see through it. Glass headlights like the celica's on the other hand shouldn't do that. I'd have to assume that plastic is porus compared to glass which would make it absorb something that turns it hazy where as glass might not? I'm not really sure. Just a guess. >

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