Not quite understanding what you are looking for. I didn't know these trucks had a trailer tow plug back there, the most they ever gave you was blunt cut wires. There are aftermarket companies that make tee adapters where you unplug the original harness, plug the tee in place, and then you have a spot to get the lights hooked up for the trailer. But no electric brakes.

 

 

 

Do you have electric brakes going back there? Or is it just lighting? Several of the connections on a 7-way would be empty if you are just running lights.

 

 

See this is what I don't under stand?
If you have 4 wires going into the 7 way connector you would have 3 connectors not used.
What do the 4 wires supply? I can see 3 right turn / brake, left turn / brake, running / tail lights. So what is the 4th maybe a ground?
And what 2 are spliced together to make 6 way?
Now if you have electric trailer brakes that would make 5 connections if that 4th wire is a ground and it should be.
On some 7 ways they run 12 volt power on 1 connector to charge a batt. that would be in a travel trailer, that would make 6 connection
The last connector is hardly used but some use it for back up lights on a trailer.

Now to this harness (4 wires?) that you say runs from the front to the rear that you are looking for and not sure of the part number I wonder if this is the factory trailer wiring harness that connects a relay for the running / tail lights?
I know our trucks have a yellow heavy gauge wire in the engine bay side by the brake booster area. I am told this wire is switch power for the relay that this trailer harness plugs into.
It could have also been used to switch power a relay for roof lights & fender lights on duel wheel trucks. I could not under stand how this all worked.
If you do a search on here I think for either trailer light relay or roof light relay or yellow wire by booster there was talk of how this all worked and may have the number you are looking for.

BTW the reason for this relay feeding trailer lights is to take the amp load off the head light switch. When more load wad added to the switch either roof cab lights &/or trailer lights it could over time weaken the breaker inside the switch and trip it. This was always fun when driving down a dark road pulling a trailer and have ALL LIGHTS go out!
Dave ----

 

Can you post up a picture of this "the male socket plug that plugs into the factory 4 wire trailer harness"?
Dave ----

 

In this link for a 86, if you had a full blown factory trailer option, apparently you did have a connector back there for the trailer plug. But looking at the diagram, after the connector they still gave you blunt cut wires that you hooked to your proper trailer plug. The connector is c1110 and c1105

Trailer Option - ???Gary's Garagemahal

The above link has a reference to a picture of these connectors on listed on page 75-9. The link below has page 75 in it.

Exterior Lamps - ???Gary's Garagemahal

P.S. The picture above shows c1105 as a open plug. The other one has wires already in it. They changed this year to year, so you can't go be this for every year truck with this option.

 

 

 

E3TZ-13A576-B .. Trailer Lamp Feed Wiring Assy / Marked: E3TB-13A576-AA / Obsolete

1983/88 F150/350 with trailer towing and/or Camper Special package / 1983/88 Bronco with trailer towing package.