Remove the grille to make it easier to get to the whole lens.
Buy and use this, following the instructions:
https://www.amazon.com/gp/product/B001AIZ5HY/ref=oh_aui_search_asin_title?ie=UTF8&psc=1
Then seal the lens with this, or some acrylic spray clear gloss.
https://www.amazon.com/Blue-Magic-7...1-5&keywords=headlight+sealer#customerReviews