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It is possible that you have already fixed these issues, but I am going to make one simple recommendation, and show you a couple of examples, that I hope will help you.

Go back to basics. This is 12VDC and the trailer wiring is normally sized to transmit that voltage properly. You have a least 2 wires that are actually abount half the guage they should be from your photos. The two wires specifically in question are important to sensing the connection.
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Your brake controller wire is cut about halfway through. Note whoever cut the sheath sliced through several of the wire's insulation.

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Your junction box also has an issue. Your battery power wire is using an improperly sized terminal connector for the wire guage, so half the wire isn't in the crimp.

Finally, make sure the ground is firmly attached in the junction box and on the trailer frame.
 

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I agree with all your statements. Unfortunately I have already fixed all those issues.

I removed their small ones and used the correct size wire terminals on the ground and 12v wires. I also cut back the main harness and re-stripped everything.

I then ran a dedicated 12ga ground loop and blue 12ga from the j box all the way to the first brake magnet. Added a good trailer chassis ground (10ga) by sanding down the powder coating to bare steel before attaching the ring terminal.

I also replaced the 7way trailer side plug with a new one.

I should have taken an after pic of the J box. It's much better now but the issues persist.

This pic is a semi before pic. You can see that they had no rhyme or reason to the trailer wiring colors. I fixed all that and made things the right size. I also soldered, greased then heat shrunk all connections instead of those crap T tap splices they use.

The wire going out the top of the pic is a new 10ga chassis ground. The white going to the left runs all the way to the first magnet, picking up other grounds as it goes. They ran Blue to White but I ran a new brake power wire as well in Blue.

The black had the wrong size terminal and they had over tightened it on the stud and stripped the stud out of the plastic j box. I had to use vice grips to pull it out. I then re-epoxied the stud into the j box.

This trailer doesn't have a battery charger so I haven't connected anything to the 12v wire.

It was a mess

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Good deal, but stick with the theme. If the truck is telling you the trailer is disconnected, it knows it was connected. You have some basic power flow problem affecting the sensing. Somewhere between your controller and your trailer brakes, you probably have a minor open condition. The controller should be attempting to sense the magnets in your brakes. Could be a frayed or poorly connected wire anywhere in that system. The relay could be bad. Yes, the controller itself could be bad, but that is probably the most costly peice to rule out. The others are magnets, connectors, wires and circuit protection devices. The 12V wire is to charge a battery (which you still may not have), not for a charger.
 
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