If I can located the proper connector and pins it isn't anymore difficult to install the OE controller. The brake controller wires are right there on the upfitter switch pigtail.Having just gone through the research on this you are way better off installing an aftermarket brake controller. I installed a Tokonsha primus IQ. There is a YouTube video that walks you through it on a 2016 Transit 250.
I just need the proper pigtail or connector/pins to make my own pigtail that includes both upfitter switches and brake controller. Or I try to find a wrecked Transit with both options and grab the pigtail harness. Doesn't seem likely.