On the E39, there's a harness you can tap into under the center arm rest. I believe it's actually for the factory phone. There's a mute wire and speaker inputs. I did this on my old 540 with a Parrot CK3000. I did a little searching on roadfly but I can't seem to find the wiring diagram. I'd assume the E53 is the same.

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Matt Atkins
'01 Titanium Silver X5 4.4
'01 Lemans Blue M5