That's a bugbear of mine too, I keep forgetting to turn off and turn back to Left Mirror for reverse dipping...another grumble lol
My Dad's old Citroen (V plate) used to just dip the mirrors and my old Honda (06 plate) heated mirrors were great. If outside temp was 3c or below the heated rear window would come on automatically as well as heated door mirrors and after 10 mins go off. If I turn on the back window manually, same again! Can't believe in a modern car that's not automatic! Can't have everything I guess but something that uses fuel, all be it tiny amount, you'd think it would do that right!