Both mirrors share a common power source- fuse # 30 in the under-dash fuse panel, which is only live when the rear defrost is on.
The power wire to the left mirror is Dark Green/Purple stripe at the 20-pin connector inside the driver's door to the memory/heated mirror. It should provide full battery voltage to the heater when the defrost is on.
There are three Black wires in that connector, all going to a common eyelet'n'bolt ground, G300, which is under the carpet at the bottom of the driver's "A" pillar. Remove the stepplate and the kick panel to get to it.
Chances are pretty slim the problem's down there, because that's also the shared ground bolt for the power windows and locks, among other things. They work OK, don't they?
Check the DG/P wire for power at the mirror connector.
Measure continuity to ground at the black wires.
If you don't have any or all of these, the first place to look is inside the soft corrugated tube between the door and body. It's very common for random wires to break in there due to the constant flexing of the door opening and closing.
If you have power and all grounds, the heating element has gone bad.
If that's the case...hoo boy!
The replacement GLASS (electrochromic) with the heating element built in is ~$350.oo from Ford - part # F7ZZ-17K707-CA.
That's not the worst; a complete paint-to-match mirror assembly is $1175.oo (up here in Canada)!
Let's hope it's wiring.
