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The XM module is located above the glove box the antenna lead for XM plugs into this module. The antenna you currently have uses the base for XM and Onstar GPS, the little dingy antenna off the top of the base is the cellular communications antenna for Onstar cell phone usage.
If the old style mouse (or the shark tail) antenn works, I would just install it on the other side of the vehicle, run it into the roof rail, then down the black A-pillar molding, inside the vehicle and attach that to the XM module box. Just leave the current XM on the vehicle and leave the other leads plugged into the Onstar module.