What is the square thing built into the dashboard to the left of the radio and to the right of the steering wheel 

From the manual of the 2005 Cadillac CTS: There is also an interior temperature sensor located to the right of the steering wheel on the instrument panel.  These sensors monitor the solar radiation and the air inside your vehicle, then use the information to maintain the selected temperature by initiating needed adjustments to the temperature, the fan speed and the air delivery system. The system may also supply cooler air to the side of the vehicle facing the sun. The recirculation mode will also be activated, as necessary.  Do not cover the sensors or the automatic climate control system will not work properly.

Interesting... I'll have to look at my sister's '06 STS, don't recall seeing something like that.

I remember seeing a strange little vent of some sort similar to that to the right side of the center stack on family Ford products--Rangers and Explorers.  Always wondered what that was for.  A vent for the airbag (which had a separate cover on 90s models?)

 

Edited June 13, 20206 yr by Robert Hall

Maybe.  Probably @Drew Dowdell  knows.  I believe he owned one.    

But what @riviera74  said about it being a temperature sensor sounds just about right. 

Yes @riviera74 is correct, I had that on our CTS that my wife drove. The salesman made sure to point it out and remind us to not cover it as the wife was thinking of putting her cell phone holder right above it at the time, but the cell phone would cover it and that would have affected the internal temp.