Over the weekend I wired up my seven way plug for my trailer.
After much though. Tinkering. Test light. Screaming. Frustration. Fuse blowing. And Praying.
I finally got it.

My 1 question is, if I'm honking up to a 7 way plug, why in the world is there 8 wires ? I can't figure out what the last wire is for. If it's important or what. What I do have is a fully functional plug. But just curious as to if anyone knows why there is 8 wires coming into the stock plug on the pickup side.

What I have is

Ground
12v power
Backup lamps
Left turn / brake light
Right turn/brake light
Trailer brakes
And running lamps.

 

If you were to give what make, model, and year of truck you are talking about, we might just be able to find a wiring diagram and answer your question.

If you were to give us the wire colors and the tracer colors, that would help a lot, too.

Pop

 

2002 f250 7.3. 6 speed manual

I'd have to get back to you on wire colors. They're hard to make out.

Off the top of my head
Big yellow wire was ground
Small yellow left blinker
Brown right blinker
Green wire
Dark blue maybe purple
Black with stripe trailer brakes
There was another big wire
And a small one that looked black but wasn't used for anything that I can find

 

What happened was I went to the local parts store (there's actually 3 of them -napa, oreilly, and EL auto parts) anyway, I was just looking for a plug and play type 7 way that would just connect to the stock wiring harness plug on the truck side for the trailer 7 way.
Well nobody could come up with anything so I decided to just cut, strip, and hardware the plug in myself. !
Anyway as stated before I did figure out everything and it's working great. I just have that 1 lone 8th wire under the truck that I don't know what it's for

 

Big black wire goes to acc terminal and is used to charge trailer battery. Other wire may be back up lights on trailer. I replaced my plug a couple of weeks ago and used this diagram to make sure I had everything connected correctly.

Trailer Wiring Diagrams | etrailer.com

 

When I did mine a few months ago I got a harness from Napa that plugged in between the taillight harness on the truck. The extra wire on that harness was to go to a ground on the frame of the truck.

 

What I did was I cut the stock plug on the truck side for a seven way connector. There's a big plug with eight wires and a small plug with a couple wires for the taillights.

I couldn't find any plug-and-play kits at the store so I just decided to cut the big plug and hardwired it in. No big deal I'm just wondering what that extra wires for.

Like I said I already have 1 left turn 2 right turn the 3 ground 4 reverse lights 5 12 V power 6 running lights 7 trailer brakes.

 

Here's that wiring diagram:

Pop