OK, anyone that may be wondering. The stock XM antenna on the GRand Prix works great with Sirius radios.
The control box is near the drivers side taillight behind the carpet.
I removed the XM antenna from the control box and cut the end off( leaving enough to resplice later if needed).
I then took my magnet mount Sirius antenna and cut off the antenna.
Reroute the sirius antenna down to the XM controller.
Strip the wire back on both and splice the center wire of both together, then splice the braided wire together. Cut off the foil shield.
Make sure the braided wire does not touch the center wires. Tape up or heat shrink.
Enjoy stock antenna with any sirius radio.
