The problem with plastic lenses is similar to the old enamel paints: over time & aggravated by UV light, the resins evaporate to the surface. Some resins also yellow with age.
What I've done with customer cars is to wet-sand the lenses with 1000 grit, and then polish them with rubbing compound using a small pad (mask around the lens). That helps a lot, depending on how bad they were beforehand.

Cloudy lenses also reduce light output; that seems lost on a lot of people.