I have a 2006 Harley Electraglide and the inside of the headlight lens is smoky looking. Any ideas on how to clean it? The back of the headlight is plastic, so I guess solvents are a no no.
Headlights are generally acrylic plastic so solvents aren't a good idea as you rightly say. A smoky/foggy headlight can often be polished back to clear. There are purpose made plastic polishing compounds, they don't cost much and in my limited experience they work well. I've also heard that you can polish with a baking soda paste or toothpaste, of those 2 toothpaste makes more sense to me as it is actually a polishing compound. Plus, your headlight will be minty fresh!
Before you polish clean the headlight surface thoroughly with soapy water, you may want to soak it in vinegar for a few hours as well as that will dissolve some of the crud without damaging the plastic.