I installed the Nissan 1400 brake sensor, soldered the existing wiring harness onto it. The warning lights immediately started flashing again. I then removed the switch and tested it, seems like the sensor switch doesn't open and close correctly, cheap R110 part, so no big deal.... The switch needs to be in the "Open" position when the diff lock is off - i.e, when the switch is fully depressed, and in the "Closed" position when in diff lock mode. When the switch is completely disconnected the warning lights are off, which confirms that this is correct. Strangely, the old rusty switch I removed is stuck on the "Open" position - i.e. no continuity in the circuit. Seems there may be an earthing problem somewhere else causing the fault. I will order the genuine part so long from Nissan and start searching for the earth fault...
The Actuator does work, but there seems to be some dirt inside as it feels and sounds a bit rough. Water must have got in there at some point. Will try and carefully clean it with a pipe cleaner and some Q20...
More updates to follow...