Welcome to the Forum.
Are yiu referring to the +, - on the shifter lever? For manual mode on the Allison?
What year is your Duramax?
Do you use the tilt steering wheel a lot? Meaning every time you get in or out of truck?
I remember reading about some wires at the bottom of steering column were cut from the usage of the tilt wheel. I am just shooting from the hip. But its worth a check.
If its something different I would next go to the switch on the shifter.
Report back and let us know.