What Sirius Antenna are you using? I am using a TERK SIR Mini inside my E38, behind the windshield and under the rear parcel shelf. It works perfectly. Most folks have mounted antennas inside the E53 spoiler without problems. Have you tested with a different antenna? Or alternatively, have you tested with you current antenna mounted externally?

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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