Are you sure it's on the inside? I'd try to use a plastic cleaner (Plasic X works well) before trying to take apart the headlights. You have to actually bake them in an oven to remove the lens from the assembly.
I'm with Joe. Most people who put film on it they put it outside. Try a heatgun and peel it off. If it is inside it is most likely paint. Why would somebody do that? Kind of defeats the purpose of the headlight.
Bought some Meguiar's clear plastic cleaner. Worked great. The film ended up being on the outside, but windex wouldn't take it off. Looks like new now.
That stuff does work very well. A lot of places are offering "headlight restoration" services now where they take your old opaque headlights and magically make them clear again. They're actually just doing the same thing you did and charging folks ~$50 for the simple procedure. Spending a few bucks for this cleaner and several minutes of time each year will keep headlights looking clear and new indefinitely.
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