A general note on this thread. I also have had brake issues, electrical. Once I found that 3 of the 4 wheels had no working brakes, due to corrosion issues where the magnet wires connected to the brake wires.
The easiest way to check for good brake electrical is to measure the total current going to the brakes. It will be between 2.5 and 3 amps per wheel. So a single axel trailer should measure 5 to 6 amps, and a double axel 10 to 12 amps.
Just identify which pins on your cord which plugs into the tow vehicle has the 12 volts on it and which pin is the one for the brakes. Put an ammeter between those two pins and you will measure the maximum brake current.
Some brake controllers have an ammeter built into them, and you can read the current that the brakes take directly.
The Humming sound mentioned in one of the posts above when the brakes are activated from the brake controller is caused by a rapid on/off cycling of the power to the brakes by the electronics in the brake controller. Not all controllers operate that way, so it is not a universal thing you will find (humming). When the break away switch pin is pulled pure 12 volt
battery power is applied to the brakes so no humming will be noted then.
The best wire nuts for brake use are the type which have an internal "goo" in them which keeps water out. They are designed for wet locations and underground splices. You can generally find them at Home Depot and Lowe's as well as a well stocked hardware store. Under no circumstances use regular wire nuts and fill the ends with silicone. That will eat the wires up really fast as the normal silicone acts as a corrosive to the copper. Also, do not tape the wire nuts with electrical tape as that only keeps the water which will get in, in, and will not allow it to drain out. Use the special wire nuts or crimp or solder connections covered with water proof shrink wrap material.
I hope this has added something to the thread of use.