I put one of the parrot kits in my Ford truck, and there was no mute lead either. I had to go the "universal" install route which is to splice into each speaker output lead (2 fr. & 2 rr.). You splice the kit's relay box in-line with the output leads from the radio, and when you get a call, the relay cuts the speaker output and allows the phone call to be heard through your front left speaker. It's a little messy, but works fine. It should work with any system. Caveat, if your X is still under warranty, I'd avoid cutting any wires or voiding warranty issues.

__________________
'06 X5 4.8is, TiAg/black, cold weather pkg., comfort seats
'07 750Li, TiAg/black, sport pkg.
'05 Ford F350 silver met./slate gray, Lariat, 6.0 PSD, CC, DRW, FX4