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The XM signal is received by the radio head via two wires that go into the back of the smaller of the two radio connectors.
This is the black 12-pin connector, and in terminal H of that connector the right audio signal from the XM receiver is received, terminal J is the left audio signal from the XM receiver, and terminal K is the audio common signal from the XM receiver.
So, if you looked at the back of this connector and there were three wires coming out of these three terminals (D/GN-WH-Terminal H, BN/WH-Terminal J, BK/WH-Terminal K) then somewhere, just above the glove box, is a 16 pin, black connector that will connect to the XM receiver.
The XM signal from the antenna is NOT received by the radio, it is received by the XM receiver module, located above the glove box, and this receiver module sends a low signal to the radio head and the radio head interprets this signal, and then plays XM radio.
The factory systems work only with the XM receiver module.