You need a Sirius module. If you had the Sirius module installed, you would get a SIRIUS option when you cycle through the inputs with the MODE button. It sounds like your car is pre-wired for Sirius, but not fully installed with Sirius.

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2000 E38 750iL Highline Edition, Upgraded with the lastest 2006 X5 Electronics
MKIV Navigation, 16:9 Display, Bluetooth, TV/Video with Backup Camera, Sirius, and Aux Input