Check otu the Online WDS link above. That has the wiring diagrams availabe for the X3.

With the X5 audio system, you can not easily add a line converter if you have the DSP option. I am not familiar with the X3 audio systems, but it could have some of the same technicial limitations as the X5 systems. Therefore, I would research the line and speaker output options on the X3 based on the different audio system options. Other members might have specific X3 experience.

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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