[DirtyLarry]

The service manual is less than sterling with testing the heated mirrors. It mentions checking the heating element supply voltage circuit (which would be infinite resistance on a DVOM) for a short to ground but that is about it.

It does say to verify if the rear defrost is working (system check) first then check the HTD MIR fuse, then check for poor connections if no fault is found with the previous (if the power movement function is working then the connection issue would be unlikely).

Outside of that, you may have to wait until old man winter returns again to test it. I have not heard of these failing often but did notice they change colors when the defrost is on. Does your mirror turn a greenish color when you turn on the rear defroster?