I found a way to take it to the next leval. I did get the 3M kit and I did our 2002 Coachmen Mirada, 97 Subaru Outback and 97 Honda Civic. It did work great. I had done the headlamps on the RV before and they fogged over again within 6 months. We live in S FL and often tour the Southwest and I think the sun just does them in.
Here is the trick. I got clear heavy plastic covers for all three vehicles. While these covers are usually used to "black out" headlights they also make them in clear. When getting them for the Mirada I had to locate the numbers on the lens edge and do some research. They were mainly made for use on a Polish truck but also one or two other vehicles. There is an art to installing them. They can't be touched in the inside, cleanliness is important, they use slippery mixtures of dish soap and distilled water to float them into place and the water is squeegeed on. Because the lenses are curved they have to be stretched a bit to fit which requires a heat gun. Three years later all three vehicles look like they have new headlamps. The Honda ones would have been over $600 to replace and the Mirada a similar amount. I saved a fortune and now have headlamps that are stone chip resistant!
Kahoona
Just think of it as a boat except that it is the top that needs to be waterproof.