Out with the old and in with the new.
Much of the Mustang's electrical system didn't work when I got it and as I started dissembling the car, I could see why.
The previous owners had incorporated foreign electrical components throughout the harness. They had caused a lot of damage in the process.
One thing I can't stand is a wiring mess so the best option is to start over with all new wires. I installed American Autowire's Classic Update kit on my Camaro years ago and was extremely happy with it. How often does everything always work on an older car?? The kit uses a modern fuse box with flat blade fuses and has extra circuits for power accessories commonly upgraded. The harness is made to fit the way the original one did complete with all of the control's and light's fixtures at the ends.