Good day!! Does anyone know what the button/knob on the back dash between the defrosters is?? Can't find any reference in the manual. Might be decorative for all I can find ;-). I have a 2016 3500 Cummings 6.7 if it might make a difference. Thanks so much!! BTW I love this truck. Granted it's a Laramie compared to my 2007 3500 lone star but it's a great truck!!