I have a 2005 E350. I have the OEM plug at the kickpanel hooked up to the brake controller. The factory wiring stubbed at the end of the left channel by the back of the van has a green, yellow, black, and a brown wire with a white stripe. Am I still going to have to run a power wire to the battery? And what to do with the blue wire from the 7way female plug.
I know the purple wire off of the 7way that goes to the reverse lights will be terminated (not used).
Please help!!

 

I ran into a similar problem with my 2008 E350. I could not find where the factory trailer wiring kit would plug in for the brake control and charging wire.

I ended up running 2- 14ga. wires (doubled up) to the rear from the battery with a 30A in line fuse. I really like using the double shielded 2-14ga wire because it is so easy and the extra vinyl cover is really heavy duty. I also ran a 10ga wire from the brake controler blue wire to the wiring kit pig tail. I hard wired that portion of the factory kit. I entered the cab at the driver side foot well, under the plastic trim.

The factory kit comes with a relay to power the trailer circuit seperately. You might find that not doing it that way might give you limited output at the trailer.

 

There is a round plug, much smaller than the 5 or 7 pin in the frame rail at the rear axle on my 99, I've been curious if it is part of it, maybe there's a pig tail that plugs in it that extends it to the rear. It's been said that it's wired in the box under the hood, you just need the fuse and relay installed to make it active.