Brake controller issue

[daz7 Junior Member Find all posts View Profile Close daz7]

Getting some strange behaviour from my brake controller, would appreciate some advice.
The controller has been lifted from my old 2010 GXL where it was working fine, and wired up exactly the same way in my new 2019 GX. Wiring diagram is attached.
What I am seeing is constant 12V on the blue wire from the controller, measured both at the trailer plug and the controller output side.
This causes the trailer brakes to be applied with full power all the time, regardless of the brake pedal position.
My understanding is that the blue wire should carry zero volts unless the car is on and the brake pedal applied, when it should provide a proportional voltage depending on how the controller is set e.g. 6V.
Anything I might be missing?
The black (+ve) and white (-ve) wires from the controller seem to be connected ok, the unit gets power and I think I have measured resistance at the earth point ok.
Controller is a Hayman Reese Guardian IQ.

[daz7 Junior Member Find all posts View Profile Close daz7]

Just an update: I had a sparky check it out and they found a shorting connection in my wiring somewhere and after fixing is all good now. I thought I had done it all so carefully, but maybe got distracted in the middle of something then forgot to finish it properly, who knows.