I did read your thread before I posted this one, and if I remember correctly, I think I hotlinked the image from yours lol.
But the truck wiring is what I am trying to figure out. I borrowed a buddy of mine's trailer to tow my Mustang to the tuner after I build was complete. Well with the trailer plugged in, I wasnt getting any lights on the trailer, however the brakes charged.
So I grabbed my multitester and started testing connections on the 7 pin while the wife was inside the truck hitting the different functions. I received little to no readings on the 7 pin. So I went to AutoZone and bought that 7/4 pin harness that I linked above. The AutoZone piece came with a round connecter to plug into the truck's harness. Obviously this wont work. So I cut the round connecter off, cut the wires from the dead 7 pin to use its square plug. The colors for the wires were different, however I spliced the wires together as I thought they should go, and plugged it in to both a 7 pin trailer and a 4 pin trailer. The light results were the same on both trailers, partially working, however not as needed (right turn signal wouldnt work, left turn signal made both turn signals on the trailer flash, however independently of eachother, not as if they were hazards). I did find that if the break was applied on the truck, that all lights worked as intended.
This makes me believe taht there is an issue with the truck's wiring, and possibly a ground. I did buy the $100 replacement harness with the correct plug just to make sure I knew I wired the other piece right and received the same result. We did hook the 7 pin trailer up to another truck, and everything worked flawlessly.