Do you have power at the Onstar fuse in the BCM? if so then it sounds like the Vehicle Communication Interface Module has gone bad here is the location so you can check it out and the fuse location as well. Please check the images below, here is how the module is set up once replaced. Check out the images (below). Please let us know what you find. We are interested to see what it is.
Repair Safety Notice: This information is for general instructional purposes only. Vehicle repair can be dangerous. Verify all information, follow manufacturer service procedures, use proper tools and safety equipment, and consult a qualified repair shop when needed.