For what it's worth, I just had to get the painted mirro backing replaced on mine. It was cheaper for me to get the entire mirror from a junkyard, then to get the primed part from the dealer and then find someone to paint it.
I think the piece you are pointing at is black plastic on all models, so you may be in better shape and can just get it from the dealer.
I don't have the part number for you, but I find if you call a dealer parts department, they are very good at identifying it on the phone based on your description and giving you a price. As a point of reference the primed mirror back was $120 OEM. (there were some aftermarket options online for much cheaper)