CAN'T GET IT OFF

Turn the lights on and then rotate the knob until you see the slit. In the slit you will see a metal piece. Insert a small screwdriver in there and pull on the metal piece. The knob will then pop off. It is kinda tricky, just take your time.
 
exactly what he said, and actually, you might want to disconect your batt. just to make sure you don't take too long with it all or mess something up. I'd check the actual light switch module too, they tend to go bad sometiems.
 
Taking that knob off for the first time will make you curse your mom. :bang: :bang: I finally got tired of mine and just broke the plastic to get it off. This just gave me a good excuse to get a polished aluminum one! :D