looks like a microphone or speaker?

Curious. I just bought an 05 TB LT. On the pillar between front & rear door on driver's side toward the top, there is somthing that is round and about ½" diameter that looks like a very small speaker or microphone. I have OnStar, but it wouldn't make much sense to me to mount a microphone behind the driver. (By the way, where is the microphone for OnStar?) No sound comes out of it, either, so I'm guessing it isn't a speaker. Anyone know what this is?
Thanks.

i have an '05 too and my onstar mic/speaker is right above the driver seat in the ceiling. if you dont have one there then i would expect yours is where your talking about behind your seat.. if you have a sunroof then thats probably why its there.....

The small speaker like item on the B pillar, is the temp sensor for the automatic temperature controls. This is what senses the drivers side temperature....

The onstar microphone is in the headliner, near the center of the drivers side dash...

Just wanted to piggy back what lkurek said...

It is the temp sensor.

Thanks to all for replies. I just joined this forum and it's really interesting to read everyone's opinions on their Trailblazers. I happen to like mine very much. Of course, it's brand new and I got it after trading in a VERY problematic 02 Ford Explorer. I have always owned GM cars in the past and always had good luck with them, but when the new Explorer came out, I got one. Looks, of course, can be deceiving, and, yo make a long story endless, I have owned my first and last Ford. Hopefully, I'll have good luck with my TB. Thanks again for the replies.