Heating element probably out, mine is out too, just waiting to get a new mirror (no money right now).
The mirrors just pop out if you tilt them down all the way and use a soft pry tool and pry and pray, it'll pop out pretty easy from what I read on other forums. The mirror is held on like a ball and joint type. You just don't want to break the housing that the mirror is in when prying.
As far as trouble shooting, I guess check for voltage at the wires, but I bet the heating element is out, pretty common.
Aaron