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The inner lenses are yellowed, or the outer lenses?

If the outer lenses are yellowed, Meguiars Plast-X and a buffer/polisher will work wonders. If the issue is the inner lenses, the only option is to replace. Used headlights almost always have yellowed and/or burnt inner lenses, but the occasional good pair comes up once in a while.
Tony (amagalla) has a set for sale...