Not really much you can do with it. Reason its dull and not bright is cleaning agents and other things have ate pits in it, effecting the index of refraction to the air. In other words, it doesn't draw in light.

Also, its not really fiber optics or silica that has been drawn like real fiber optics. Its plastic, where they put a coating on it that will draw light better. They are dog cheap, just buy a new one.