So the problem is the theives know exactly how to disable onstar. Tons of posts about it on Facebook. They are stealing these trucks left and right, Especially in Texas it seems. They cut the horn in the wheel well. Open the truck, setting the alarm off. Which makes no sound because they cut or unplugged the horn already. Then they swap the ECM and disable onstar. It's all done in less than 2 mins. All you get is a text message from onstar and by the time you are aware the tracking is gone.
If theft is a concern install a kill switch. That's really the best safeguard. A hidden tracker is also a good idea. I have a couple on my trailers. They are pretty cheap and the service isn't bad. Especially if you pay by the year. Not a bad idea to wire in a horn or siren into a different location. I don't know like a train horn

so when they cut the horn wires in the wheel well thinking they will have a quiet entry. They get a loud surprise.