Mirrors are heated ONLY when the rear window heat is on. From the manual, see my underline in the last sentence. Also, see the note about cleaning the mirrors (yes, they can get a little spotty, I think so anyway).
HEATED WINDOWS AND MIRRORS (IF EQUIPPED)
Heated Rear Window
Note: The vehicle must be running to use this feature.
Press the control to clear the rear window of thin ice and fog. Press the control again within 15 minutes to switch it off. It turns off automatically after approximately 15 minutes, or when you switch the ignition off.
Do not use razor blades or other sharp objects to clean the inside of the rear window or to remove decals from the inside of the rear window. This may cause damage to the heated grid lines and will not be covered by your warranty.
Heated Exterior Mirror (If Equipped)
Note: Do not remove ice from the mirrors with a scraper or attempt to readjust the mirror glass if it is frozen in place. These actions could cause damage to the glass and mirrors.
Note: Do not clean the housing or glass of any mirror with harsh abrasives, fuel or other petroleum-based cleaning products.
Both mirrors are heated to remove ice, mist and fog when the rear window defroster is activated.