If the light on the rear view mirrior is blinking red, your onstar needs servicing and has an internal problem that is triggering the light to turn from green to red. Your dealer will be able to diagnose the issue.
The Onstar antenna is located on the right side of the windshield at the very top inside the window. The antenna looks like a rectangular block about 1 1/2 tall and 7 inches wide.
I had the same problem and it is just as Allen says. There was actually some problem with Onstar recognizing my account or some such thing. In any case, it was a quick fix and works fine now. It even worked when the red light was on, so go figure.
The other thing you can do is call Onstar and have them run a diagnostic.