How do I clean the inside of the headlights?

You can't clean the inside. Truthfully it is most likely on the outside. They have plastic headlights and the sun has hazzed and browned the outside of it. So what I recomend is go to your local parts store and buy 3M 39008 headlight restoration kit. All you need is a drill and spray bottle of water and follow instructions. I have used this kit and it works great.