Where's the antenna? 2001 M3

I'm installing an XM Radio in my 2001 M3 but am unable to identify the original radio's antenna. It must be in one of three bundles I can see at the rear of the radio. Not sure of the exact model designation of my head unit but this is a description: It has a cassette tape player on the left and the BMW Nav system LCD on the right. Does any one know exactly which wire is the actual antenna wire? Through the antenna lead is how the XM signal is sent to the rest of the sound system. Thanks to everyone for a super speedy reply!