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Unless I have misread the question it seems like this is a trailer issue. You need 12 volt power on the DB wire to apply the trailer brakes, not to release the brakes. Have you double checked and verified the trailer brake control is getting the correct power and ground signals at the in cab connector.
The problem has to be on the truck, there is not poitive juice on the test light when I touched it to the truck plug but the brake controler does light up. Also, when I touched a positive truck wire to the Blue trailer wire, the brake released but the brakes won't work because the truck is not sending the info and I had to bypass the trailer controller to make it roll
Are you testing the truck harness on the dark blue wire with the brakes applied and the truck running?
Are all of these inputs correct at the controller
Have you done a ohme test o nthe DB wire from the controller to the 7 pin trailer connector.