I am stumped over this lighting problem! I need to fix the trailer lights soon, on a factory tow package.
Situation: All trailer lights worked fine a few months ago. I have owned the truck for nearly seven years now, with no problems to speak of.
Today, at the trailer 7-blade socket, and at the harness connector up under the spare tire I have everything EXCEPT turn signals and brakes. I have running lights, power and ground, though - for both trailer side and truck body side.
Connectivity within the rear harness segment is great, with almost no measurable ohms. I took the rear harness segment off, and checked all the wiring - connectivity is excellent within the harness.
Truck lights are fine. All fuses check out fine, both inside and under the hood.

Still, I do not have any signal from the rear harness connector (next to the spare tire) for the turns and brakes.

My owner's manual does NOT refer to the relay box, which is attached to the side of the main under-hood fuse box. Is this the add-on harness for the tow package?

The two relays on the left work when the turn signals are on. Therefore they must be the relays for the trailer turn signals?
What is the center one for, and what are the two on the right for?

I would like to finish troubleshooting this before I start tearing harnesses apart, so I really appreciate any help!

 

Forgot to add the picture.

 

did you figure this issue out ? i just bought a 98 with factory towing package that was never used according to original owner. it has a 7 way round plug and i put a 4 way adapter in and nothing works. checked all fuses all are good. scratching my head now ? thanks kenneth

 

HOLY MOLY DEAD THREAD REVIVAL!!!!!!! I know you said check all the fuses but there is one right behind/under the power distribution box in the engine compartment. Second thing, do you have turn signals and brake lights? Next would be to grab an ohm meter and see if you have signal coming to the rear segment of the harness. If you do, the check the blades for continuity from the rear harness segment. Take it out of the truck and then check each blade to make sure you have a signal through the harness. Make sense?

 

Today i went back and looked at fuses again. found another 6 circuit lil box next too the distribution box and found a 20 amp fuse blown. lights are working now, i never imagined with the factory tow package adding a brake controller was so simple, awesome job by ford for doing that. Thanks for your reply.

 

Anytime I just wire in the harness provided and I'm golden! Love it! The box that you found is what I was referring to I think. Glad you got it working. Enjoy!