AUX button on '99 GMC Sierra???

Hello:
I have a 99 GMC Sierra 2500 truck and have a question I've been unable to answer for quite some time now: In the center dashboard, towards the right-hand side, is a large push-button switch labeled
"AUX." When the button is depressed a relay can be heard engaging in the rear of the driver's side of the vehicle. Other than the audible "click" of the relay, nothing else in, or outside of, the truck seems to turn on. Does anyone have any idea what this button is for? There is no information regarding this topic in the Owner's manual and no one at my local GMC dealer has been able to help. Any information would be greatly appreciated!
Thank you, Ryan Winland
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nah, mass production would still account for saving $20 on a switch, they would have a plastic plate that would cover it if it wasnt an option.
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