All I can find is to adjust the mirror fully down, get fingers behind it and pull.Remove the mirror glass and measure the heating element resistance. Don't need to guess if it is broken.
Find some youtube videos showing how to remove the mirror glass, pretty easy.
My mate says the same, maybe a fuse or relay. I believe the auto dim interior mirror is working. I also think that both mirrors failed sounds suss. Trouble is that I don't know much about the workings of the mirrors and with the lack of a workshop manual or a fuse position schematic don't know where to start.
A) Where is the fuse or relay for the heated mirrors?
B) How do I remove the mirror glass without breaking it?
From a www search it looks like the W166 mirrors have two wires for heating and a round connector for the auto dimming.
Just need to know how the power to the heated mirrors is tuned on.