I'd like to add a third brake light to the back of my 2016 Minnie Winnie 27Q. Any advice on what wire in the Ford chassis wiring harness it would be best to tap into?
Looks like Ford provides a wire that comes from a connector near end of the Ford frame rail. It's shown in the body builders layout electrical section. I snipped a picture from it below.
Violet with a white stripe Stop Lamp wire looks like the one you're after. If it's like my F53, there'll be a small tag on the wire too.