Here is an answer to someone with a 1999 Ford F-250 from ETrailer.
"On your 1999 Ford F-250 Super Duty the two boxes in the engine compartment are the power distribution box (larger box) and the trailer tow relay block (smaller box) which is attached to it. Open the covers to access the relays and fuses.
You will need to look in your owners manual and locate the Trailer Tow Electric Brake and Trailer Tow Battery Charge fuse locations and insert the 30 amp maxi-fuses (provided) into them.
Also in the owners manual locate the Trailer Tow Backup Lamp Relay and the Trailer Tow Battery Charge Relay locations and insert the relays (provided) into the trailer tow relay block. Put the covers back on the boxes.
With all of the fuses and relays installed you should have full power to your 7-Way connector. I am including a link to the installation instructions on the Tow Ready wiring harness.
https://www.etrailer.com/question-3141.html
Hope this helps.....and I had the same dilemma on a 2014 F-150.
Got the missing relay and fuse inserted and bingo......full lights and full charging.
The odd part of it was the truck was equipped with the "tow package" yet the proper relay and fuse was not included.