adding satellite radio?

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Not sure about the connectors, but I believe they only ship with one type of onstar antenna, and that antenna is used for the GPS signal to Onstar, cell phone for Onstar, and XM receiver antenna.
Two ways to check. One, is there a Black 16 pin connector taped to the harness just above the glove box? This connector would have a brown/white wire in terminal 2, and a dark green/white wire in terminal 3 (it will also have some other wires). These are the two circuits that feed the XM signal to the radio.
The other way to check is to pull the radio, which is quite easy, and check the small 12 terminal black connector at the rear of the radio and see if it has a dark green/white wire in terminal H and a brown/white wire in terminal J (these are the two feed wires from the previous connector).

If the connector is there, and the wires are in the radio connector, then some where in the harness above the glove box there should be a small two terminal connector which has the leads for the antenna for the receiver.