2001 supercrew
I just went to hook up my new used boat and trailer and found a problem. At first the right turn signal and brake light did not work. So I replaced the bulb but still not working. I then rewired the whole trailer, still not working. My truck has the towing package. The trailer has a standard 4-wire plug, and I am using a 7 to 4 wire adapter. I used a tester and found that I have no power for the wire in question. I probed the wire about 12 inches from the plug in still no power. The taillights on the truck work correctly.
Does the pigtail going to the trailer connector have a connector that unplugs from the main wire harness?

 

It should. On my Superduty, the terminal end unplugs from the rest of the harness up near the rear axle on the drivers side (maybe 18"-24"?) from the actual plug. Good luck tracking it down.

 

There are a couple of fuses that have to be installed in the engine compartment for the wiring to work. There are also a couple of relays but I think those are for brake and backup lites which don't get used in the 4 pin connector. The fuse box is near the master cylinder on the fender well I think.

 

There is sort of a connector for the trailer harness. Actually it supplies the connection for the trailer harness and connects the main harness to the tail lights from the way it looks to me. The large plug that the trailer harness comes out of is on the drivers side frame rail beside the spare tire, most likely you will have to remove the spare to get to it. The second connector to that assembly is back around where the bumper assembly attaches to the frame if I remember right, been awhile since I was under mine looking at it. I haven't put my 7-way on the truck yet but I think the fuses for the basic light functions were there because the 4-way on my truck works and I believe that the 7-way uses the same wires for those connections.

The relays that are included in the brake controller harness kit are for the trailer backup light circuit and the trailer charging circuit, at least in the kit I bought.