Originally Posted by Gnarvan
When I was under the van this morning tapping into the tail light circuit on the trailer wiring harness for the license plate light I did notice something a bit odd. The stock trailer wire harness is a 4 wire. It looks like there may be a larger possibly 7 wire connector plug attatched to the frame rail on the drivers side just aft of where the 4 wire connector plug and harness departs to the hitch. If possible I would still like to find this circuit under the dash somewhere. Hopefully by the fuse box.
Yup. That's the HD towing harness connector. Relay is (or should be) in the power distribution box under the hood. It's always a pleasant suprise to go looking for another circuit somewhere, and it turns out the factory already put it there
Of course, I usually don't realize that until after
I've already run my new circuit.
PS - for running your switch to the stock relay.... Ford already has included some "pass-through" circuits that run from the connector near the brake pedal, into the hood compartment on the driver's side.