2012 BT50 brake controller wiring

G'day MOGUL
I don't have a 2012 BT50 so this is generic in nature.

The main feed to the controller can be fed via a fuse or thermo breaker unit, direct from the battery, as per usual. No worry to vehicle here. This thermal breaker is recommended over a fuse because a short will blow a fuse and hence no trailer brakes at all, but the thermo breaker goes on and off and despite the short will provide some braking sometimes if there is a fault of this nature.

The brake activation sense wire which goes to the globe side of the brake light switch is just that, a voltage level, which is telling the brake controller to be awake for brake sensing. This is nil current just a voltage level.
On your brake light switch it is possibly arranged with 4 wires. Two are the 12v in and out to the brake light circuit and the others will be for engine control related items running to the engine computer. I can't tell you which is which cos I haven't seen yours but someone else may know the colour and where it sits in the plug so as to make it easy for the correct connection.

The brake wire from the controller goes to the rear and therefore the trailer plug base and is connected as normal.

There is also an earth for the unit which will have to be earthed to a suitable place under the dash. This too doesn't take any appreciable current but must have a secure and defiinite earth.