Ford E 350 Trailer Brake Wiring Diagram. I think the printed wiring diagram is about as detailed as you can get. It took some research to figure out how to do.
The source of the information below is the National Highway Transportation Safety Administration (NHTSA), which maintains TSBs for vehicles sold in the United States. Are you talking about the Pigtail that would install under the dash, if you do not have the TBC ( Trailer Brake Controller ) ? Ford ESP can quickly pay for itself.
Wire up your tow vehicle and trailer with whatever it takes to get your trailer lights on, the electric brakes slowing down.
I used the search function but didn't see the trailer wiring diagram with the factory color codes or lists with them.
The following trailer brake wiring diagram contains detail illustration of the typical brake applied in trailers. How Brake Light Wiring Works. by Ed Grabianowski. Refer to the Towing chapter for more information.