Well lets get that receiver out of there and check the part number !!

From what I could digg up for you, there were two part numbers floating around for Sirius receivers with the two antenna input.

84-11-0-141-981 This first part # was reported not to work with NAV cars and was for in-dash radios only.
84-11-0-153-365 This second number was the receiver to work with NAV cars ... although it appeared also to work with Non-NAV cars (?) ... so go figure.

If you have the first part number, then this could be it !