Sounds like the middle button showing a windshield with heat arrows is probably your rear window defrost. If you've got side mirror defrosters, I believe they should come on with the rear window.
This is assuming that the rear window defroster button isn't part of the HVAC controls. If it is, then that button is probably for the mirrors.
This is assuming that the rear window defroster button isn't part of the HVAC controls. If it is, then that button is probably for the mirrors.