I think there are a lot of different types of Sirius antennae (sp?). Show us a picture of what you have and maybe we can help you out.
(I put my Sirius antenna in my rear spoiler)

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*2010 X5d - Space Gray/Black Nevada/Dark Bamboo|ZPP|ZAP|ZCW|ZTP|ZRC|322|328|386|4NC|4UB|655 |6FL
~Ordered 8/27; Production began 9/28 at 12:20pm; Completed 10/7; Delivered 11/4!~
*2008 535i Black/Black - Dinan Exhaust & S2 Software
*1995 318ti Clubsport - Hellrot Red (autocross/fun car!)
Past: 2005 X3 3.0i; 2009 335xi