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.
Images (Click to enlarge)
Sunday, January 22nd, 2023 AT 1:15 PM