I assme you mean the glass, not the whole assembly. Take it off, clean it, get all loose junk off of the plastic part, get that waterproof winshield adhesive. run a good bead all around, then use a bungee cord stretched across the lens and hooked onto the frame on each side. Leave for 6-12 hrs, never have an issue again. This is what I did.
For removing things that are gooey, I use a quality automotive paint prep solution that can be purchased at any auto paint supply store. This solution is designed to remove waxes, silicones and things that are sticky without affecting the substrate. Use clean rags and be generous with the fluid...don't let it dry on the surface, rather wipe it off before it has a chance to dry.