its one of two things-
1-brake controller is faulty.
had it happen to me brand new unit (made in China) was broken out of the box.. same symptoms as you. Spent several weeks chasing it an hour at a time. My son bet me he could fix it in an hour.
He pulled a working module from his truck and installed it
then it all worked and he never let me forget.
2-wiring issue with truck
could be something you created by accident or 10 years of wear and tear did it.
I vote take box back and exchange for a new..new one try that
if that fails then the journey of wires, fuses, connections, and misery must begin
2007.5 3500HD Silverado LTZ
Kennedy Custom Tune
Banks Intake and 4 " exhaust, Air Dog 150, Leveled front end, aftermarket upper control arms, tie rod end sleeves, bypass oil filter, Pusher 3" "hot side" charger tube, aFe Turbo inlet pipe, PPE deep Trans Pan with temp gage, 2 low mod for front differential disable, PACBRAKE 5000# Air Bags with remote on board air...otherwise stock
1953 Chevy truck
496 2 NOS kits
383 2 NOS kits chassis car (street legal)//