During testing surely would’ve shown this up as an issue...
Putting fiats “that’ll do” attitude aside, I think there are two things going on here.
1. is the repetitive movement putting stress on the harness, that can be tested for easily as I’m sure you know.
2. Is the material the insulation is made out of, many years back Mercedes changed the material they used on wiring insulation to a type that would biodegrade, however it started biodegrading in the cars long before it was supposed to, I don’t believe this is to quite the same level as Mercedes, but the repetitive movement, age, cold and heat all combining creates this problem.
On a bench flexing the wires back and forth wouldn’t necessarily replicate the fault, but take 6-10 years of roasting hot summers, freezing winters, exposure to the elements and being continually flexed, is what is causing the insulation to fail. Once the insulation breaks down the copper is less supported and more likely to fracture.
It’s interested that it is predominantly the hatch wiring which is high up on the car so likely to get the brunt of the sun in the summer months and be exposed to ice in the winter.
You could argue the drivers door is used many many more times but there don’t seem to be the same problems there, my theory is because the wiring to the door is better protected from extremes of hot and cold