You can try putting a test light on the terminals on the back side of the mirror. If it lights when you turn the defrost on then its the glass thats bad.
Odds are, the heater is burnt out in the mirror glass if the other side still works.
Not really. Just be very careful. I usually just pull the top inside corner out as far as i can then stick my test light in the gap. It should be a brown wire and a black wire for the heat circuits.