Just purchased a new 24' Sierra 1500 Denali, is there a way to put it in accessory mode?
Tried this < To put a GMC Sierra 1500 Denali in accessory mode, you can press the Power or Stop/Start button without pressing the brake pedal. The green indicator on the button will illuminate when the vehicle is in accessory mode.

Doesn't work.

 

are you looking for key on engine off? if so press and hold the button it will take a few seconds. foot off the brake pedal.