This may depend a lot on the car model and audio component configuration. It could be different between Old Gen and New Gen radios and cars with and without DSP. Also, coding differences for certain models could also affect which speakers get nav audio.

In my car, I am pretty sure that nav audio only comes through both front speakers. I have a 2000 750iL, but it is upgraded with a New Gen radio module.

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SOLD
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