I just came back from the dealer to ask if they've done this install and to check what their price was. They said, on another 2020 AT4 Sierra but like my truck, they have tried and could not install the passenger side. None of the functions work. On that "ShopChevyParts" site listed above, it actually states the mirrors are manual extend and fold. Also, like the parts guy said, it states my mirrors seem to have code DEZ and the mirrors are not compatible.
Yet, I seem to read on here that someone has installed these mirrors with all power functions working on the '20 AT4 Sierra. I'm really confused now, but did not argue with the dealer.
Did I misread?