The Bluetooth antenna is #4 in the diagram below & the TCU is #1. It's as easy as typing " BMW E53 Bluetooth antenna" in a google search box to find actual images...moreso if you click the images button to limit your search for pics. Some of the results will probably lead you right back to Xoutpost.
Here's a bluetooth PDF install guide found here on xoupost that shows two places the bt antenna may be installed based on how your X may be optioned:
http://www.xoutpost.com/attachments/...l-20manual.pdf
84506928461 (the antnna's 11 digit part # can be seen in the pic below):