Dazzle-
Mike ("oldecmech") is correct, and I was incorrect. I gave the correct location for a Ford F-53 chassis (post-2012), not an E-series chassis. No help for you at all!
Here is a link to a Ford document. On page 14 it shows the rear of the dashboard and the connector, (temporarily) attached to the rear of the coin tray in the dash.
If you have a Tekonsha trailer electric brake controller (such as the P3,
link here), you can buy a plug-and-play harness for it (link
here).