Originally Posted by dwightdi
... You should not have constant voltage on the brake actuator pin.
I do not believe that is entirely right.
Brake controllers work by limiting the current to a given load. But the initial voltage will be ~12 vdc. A current-hungry load will then drop the voltage if the current is limited.
Most voltmeters have a high input impedance which means they will not drag down a circuit with any appreciable load. A brake magnet has an appreciable load. I would expect a voltmeter to read 12 vdc at the brake wire pin if nothing else is connected.
A better test of the brake wiring would be to check voltages with the trailer connected to the tow vehicle