Electric trailer brake controllers
	
	
		Have found that there is a difference in how the electric brake controllers have to be wired depending on whether you have "electric' brakes, or "electric over hydraulic" brakes on what you are towing.
It seems that the latter requires a constant feed to one of the wires (to keep trailer brake battery on charge), and that may be lead 2 or lead 5. 
As my electric over hydraulic trailer is sometimes loaned to those with controllers set up for "electric" (only) brakes, I had the auto sparky wire me up a connector plug where he had bridged the reverse wire to supply constant power.
BUT. When reversing the trailer they have to remember to unplug the connector as the brakes come on when they select reverse and cannot reverse trailer.