If you are feeling computer confident, you may need to edit the phone's Bluetooth gain setting. Basically, there are some internal settings on a phone that control a number of things on the phone. Do a search on Motorola SEEM editing and gain, and you will see what is involved. You essentially have to hack the phone.

Also, www.bimmernav.com has an amp for the BT speaker output. That is another possible try.

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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