JBarca, most excellent info!
Kind of hard to fully describe trouble shooting with only words.
I will echo the sentiment of the OEM wiring, while it is within the minimum proper specs, it simply is dropping way to much voltage over the distance. Add the wire resistance plus any resistance from corrosion at the splices often results in poor under performing brakes.
In the case of a single axle trailers the results is no brakes since the controller turns off the output if it does not recognize the return pulses...
Upgrading the wire is a bit more work but really pays out in better performing brakes, well worth the effort.
Some folks with a single axle have turned to adding a extra brake magnet (wire tying it to the frame) to prevent the controller from turning off. While it works, I am one the fence as to whether it is a good idea or not since it will fool the controller and yourself into thinking nothing is wrong.. But if it keeps you from not having brakes at all, it might not be a bad idea either.