It just occurs to me that it's bad design having a ECU fuse go on a bad trailer...
The way I'd like to have my trailer:
1 nice fused (~40A should do) 12v to the back.
tap steering wires from all lights needed.
use these steering wires to steer sold state relays that throw 12+ to the corresponding pin on the socket.
One might have one of the rear lights and a brake light over a 2nd 12V line + fuse to for safety.
Most made mistake is having the socket upside down forced in.
If you would connect this the other way around (which fits apart from a tiny plastic thingy) would basically put 12V directly to ground when you hit the brakes. This is why it is so important to check the left / right indicators seperately. Alarm lights will work 'the other way round' but blinking left wil blink right on the trailer. You then know the plug is likely upsidedown.
If you don't have the foglight on, a non functioning RH Taillight might also be an indicator.