You could place a voltmeter across the wires for the mirror heaters.
Switch the ignition & rear screen switch on - you should get over 12volts.
If you haven't go t a meter you could try putting a bulb across the wires, a 12v bulb ( indicator bulb,stop light bulb etc) should light up if you are getting power.
As far as I know, replacing the glass is a matter of just being very careful - push gently on the inside of the mirror, not too close to the edges.
WILL TRY OVER THE NEXT DAY OR SO THANKS