I don't have first-hand experience, and I can not get my BMW software working. So, this is just a guess... BMW uses the same microphone in the E70 X5 as it does just about every model going back to the early 90s. It snaps into a little plastic cover that typically snaps into either the headliner or sunroof panel on most cars. So, the trick is to find the mic-- it is a little plastic grill piece that measures about 1-1/2" x 3/4". If you find that, just gently pry one of the shorter sides until it pops free. I think that the rearward side of the piece is what snaps out first.

Here is a link to the BMW parts database to give you a basic illustration. It just doesn't show you where in the car it is actually located.

RealOEM.com * BMW E70 X5 4.8i Single parts f hands-free facility

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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