Depends on the kind of haziness. Though some companies offer clear plastic cleaners - such as MeGuiar's Plastix - in most cases - in our cars - the haziness on the headlight lenses is caused by the innder vaporization and deposits on the inner surface of the lens, that can be removed only by disassembling the headlights, checking whether the issue is caused by non-hermetic sealing, and subsequent cleaning the inner part of the lens. To make a long story short - way too much PITA. I decided not to do it with mines...
