Hi - 11.4V should be fine for the circuit, the three pins are ground/supply/signal so if you have a good supply and ground then the signal could be the fault however that's generated by the sensor so that wouldn't explain the lack of ticking. That said the tick test should be used as a crude test though, its not conclusive so a VCDS scan would be recommended.
Wiring wise there is a joint in the tailgate area and then its strait down to the controller although I've only ever seen faults with the wiring at the sensor end, normally within about 10cm of the connector where it loops in from the loom. So check here carefully as its also possible the insulations just cracked and allowed water in to corrode the cable core.
If you do want to locate the connector then lift the carpet and underlay from the tailgate end. The loom enters the cabin almost centrally and then runs off to the passenger side. You'll find the connector under the underlay and its 6 pins as the supply and ground are common across the sensors. If you do locate it then yes, another check you can do is to measure for continuity and high resistance between the loom connector and the sensors connector. If you let me know which one is causing the problem then I can tell you the wiring colours you need to check.
