2004 Overhead Console Switch Question?

I pull into the parking lot this morning at work and the owner of the Chevy truck I parked next to gets out to ask me a question. He says his wife's 2004 AV has a button (Maybe slider.) on the overhead console above the rearview mirror that they cannot figure out what it does.  My 2002 doesn't have that button so I do not have an answer for him.

Can someone on this forum please enlighten me so I can pass on the information to him?

Thanks in advance for your help.

He could be talking about one of several different things. The overhead console may have:

2 Push button switches for the map lights
1 Rocker switch for the sunroof
3 push switches for the Homelink system (allow programming of garage door openers)

And, here is the biggie....

Not to be confused with a switch, on the left side of the console, there is a black object that may look like a switch. It is NOT. That is the microphone for the On Star system.

Here is a link to the owner's manual:
http://www.chevyavalanchefanclub.com/resources/2004/owners_manual.html

Thank you Dalefan for the informative post, I will relay this to the individual that asked the question.

Have a great day.

One other thing, if he says he thought the round "speaker looking thing" above the driver's head was the On Star mic, let him know that that is a temperature sensor for the automatic climate control.