I started with a 7-way tester.
I used the tester to confirm my tow vehicle plug was all good.
When the tow vehicle tested okay, I removed it from the process. I approached the challenge by dividing the elements into three zones . . . tow vehicle, cord, and trailer. I used a small 12 volt
battery charger, a 12 volt
jumper light, a small multi-meter, and the 7-way tester. I used the tester to confirm the trailer plug was wired correctly, and to confirm the cord was wired correctly.
I removed all bulbs in the rear lights and replaced them all after cleaning and polishing the sockets with a Dremel and cleaning and reattaching all the grounds. I replaced all the running and clearance lights with new LED lights, on new gaskets on new grounds.
I installed a new fuse panel with all new fuses. I rewired a new breakaway switch while I had all the wiring exposed. I soldered all the ground wires together with my map gas torch and established a new ground attachment point on the trailer frame. When I tested the system with the tow vehicle after testing it with the battery charger and the 7-way tester, everything worked.