We don't know if your amplifier is turning on.
We don't know what is connected to the amplifier inputs.
Please post a picture of your head unit wiring.
I've done many head unit installs, and I wouldn't do one without a voltmeter. I wouldn't work on a BMW without a voltmeter.
The most common problem with head unit installs in BMWs is powering the wrong lead for amp turn on.
So are you retaining the head unit in the car?
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High-performance audio for BMWs