the lens has build-up? are you talking about condensation, or something else? if it's condensation, all you'll need is a bottle of clear silicone (or siliconized at least) caulk. just look at your headlights, and anywhere you see a gap between the lens and the housing (even a small one is enough), squirt some caulk into the gap and let it dry overnight.
if you're talking about build-up like from waxing, then you can just use a paint cleaner (Mother's or Meguiar's are both good) if you have glass lenses. if they're plastic, then you're kinda SOL, cause anything that will remove the wax will probably remove parts of the lens as well.