To answer your current question from some digging about on the internet, I'd say that if the wiring is the issue and the gauge pegs at the hot end of the scale, you are 'missing' a ground somewhere.
Since this is a persistent problem, the vehicle isn't actually overheating and you have confirmed both those sensors aren't responding, my money is definitely on the wiring/connectors/fuses.
I was going to suggest logging to see what coolant temp came up to ascertain if the temp sensor was working and the gauge cluster was faulty. I doubt in this case you'll get a temp reading at all if the sensor isn't responding. And again since you have tried numerous clusters with the same result my money is on the wiring/connectors/fuses!