I had one installed in my 2012 F150 by the local Ford dealer and as I recall it wasn't terribly expensive. An acquaintane recently had one installed in a 2015 but it didn't work and he had to go back. What hecwas told was "oops, we forgot to tweak the software so that the truck's brain was aware there was a brake controller installed."
I would say if Ford's price is too high, try an internet search. I found OEM towing mirrors on eBay for 1/2 price. With a 21, you will probably want extended mirrors, whether OEM or after market.