I have a 2002 GMC Sierra 4X4 and it has a Aux. Lamp button beside the keyhole that turns the pass. side airbag off/on near the climate controls. Beside the light switch is my fog light button and cargo lamp button which turns my third brake light lamps on so i have no idea what the aux. lamp button does, hopefully someone knows what it does.

 

Probably prewired to run lights on the front on like a bull bar or to run a set of lights for reverse lights.