i have a 98 expedition with stock towing package...it has that round 7 pin socket, when i hook up the trailer, i get running lights, thats it, no directional, brake or reverse lights. I used a test light in the socket itself and only have power to the runinng lights, the rest show no voltage, but the truck itself all lights work, where do i start?
I have confirmed the trailer lights work fine on 2 other vehicles

and i have checked ALLLL fuses in the ****pit and underhood, all are good

 

If your 7-pin connector is hooked to 2 connectors in the left rear bumper area ( about 14" or so) it is using the factory tow. If so you will have a battery charge relay, fuse 19, fuse 20, and reversing lamp relay in the battery junction box under the hood. Of course you need a brake controller with a pigtail plugged into connector but that would only affect the brake line. The white ground wire from the 7-pin connector goes to the frame near the connectors. A dirty connection can cause strange problems. I have a '99 Expedition wiring diagram book.