Here is "how" the injector wiring on the CDTI (Z19DTH) fails. After replacing the wiring on #3 today I carefully cut open the old connector housing with a hack saw to extract the wiring connectors and inspect them.

To share my findings with you all, I did a little video. One of the old connectors had so much play in it, no wonder it was not making good contact all the time and misfiring!! The jaws of the new connectors are so tight in comparison and therefore really grip and make good contact with the pins on the injector itself.

It doesn't fully explain the root cause (could be heat stress, vibration etc) but at least i know exactly what the problem was. All the wiring on the injectors was original and therefore the new style connectors/housings came from the factory on my engine. So I think we can conclude with some degree of confidence that even the new style connectors will fail eventually; in my case 5.5 years or 96K miles!

Here's the video: