| antenna available?
Looking in the MINI Motoring Accessories 2005 catalog there are several components,
Sirius Satellite Radio Receiver 84 11 0 150 535 $275.00 (all models but not convertible) AVAILABLE
Sirius Satellite Radio Antenna 84 11 0 150 524 $75.00 (2004 & earlier) (did not check)
Sirius Satellite Radio Antenna 84 11 0 393 780 $90.00 (2005 & later) BACKORDER
Sirius Satellite Radio Install Kit 84 11 0 150 534 $45.00 (all models but not convertible) AVAILABLE
presumably you need receiver, antenna and install kit to do this but as of this afternoon the dealer computer is showing the antenna as not available. | 
