Okay, I have a suggestion. I remember a few years back having an issue where the compass heading at the top of the driver display did not display a heading and might have said CAL some of the time, and there were a couple other indications I can't remember clearly, though probably the OnStar light was not lit (should normally be green).
IIRC I believe it was fuse F9 in the left side of the instrument panel, but look in the fuse blocks (or the manual diagrams of the fuse blocks) for any related to OnStar. With the car off, yank the fuse, wait 10 seconds to a minute, and plug it back in. This should hopefully reset the related module. (I don't have the big maintenance manual (yet), but my impression is that the OnStar module is basically the circuit card of a cell phone that has a GPS chip of some sort on it that they are using for GPS position; anyone who actually knows please correct me if I'm wrong.)
Hope this helps a little.