How do you get the rear view mirror off? I'd imagine there's a set screw in there. Is that right?

 

Some mirrors have a set screw and some do not. If you don't see one then you need a thin flathead screw driver (or other similar flat piece of metal). Shove the implement into the slot on the bottom of the mount and push towards the roof of the car. With your other hand gently try and slide the mirror up. There's a little metal catch in the mount which when spaced by the screw driver will let it slide up.