Yes, the doorhandle and hatch wiring loom have been much discussed on here. Fortunately I did not suffer any significant problems in the 6 years I had my 500. Very difficult to know what percentage of cars are affected. Similarly, many VW group cars up to about 3 years ago suffered engine destruction due to sub-standard timing chains, but there are no figures on how many. Think I'd rather have my doorhandle break!

So how do you fix it

If you was to search this forum you would find that broken door handles are a very common problem on fiat 500s, handles break & hinges snap thats why there are so many available on e-bay etc . It has been happening since the car was introduced in 2008 and is still happening today ! lube the hinge end with WD40 and make it your practice when opening either door to brace your thumb on the keyhole end whilst pulling in the latch this stops a lot of the flexing that causes breaking! there is a good you tube video on how to change it yourself its not difficult if you are half-competent with a spanner!.