When I did my exterior towing lights, I rewired the 7-way trailer plug. I used a 7-way tester with a jumper to wire the trailer plug to function, not wire color. I quickly discovered that the wire colors on my trailer didn't match the color codes in the repair manual, thus, they didn't match the wire codes on the 7-way, and the 7-way didn't match the colors of the wires in the new molded end umbilical.
But everything did match the function pins on the tester.
My blog shows some of what I did.
I used a multi-meter, a small battery charger, and a pin type 12 volt
jumper light. I jumped the test light one pin at a time to feed into the 7-way and established that I had the 7-way wired to agree with the tester. I used the tester to confirm the tow vehicles were wired correctly. Then I used the tester to wire the loose end on the umbilical. Those umbilical wire colors were different that either the trailer or the 7-way, and I had the backup lights crossed with something else. It was easy to see which wires I had crossed and when I corrected those, everything worked.
Before I started on the 7-way, I replaced all the clearance lights, removed and cleaned all the rear light frames and sockets, installed all new bulbs, installed new lenses, and verified everything worked by the charger power one wire set at a time.
Two things that "looked" the same were the brakes and the brake lights. I objective tested that by jacking up one wheel and giving the up wheel a spin while plugging in the battery charger (that powered the positive brake wire) to a drop cord laying beside the trailer. Did I mention I work alone?
Once I knew everything worked and which wires were which, it was just a time thing to finish up on the 7-way and the cord.
It was during this 7-way rework that I discovered that the previous owner had disconnected the breakaway switch. I installed a new switch at this time as well.