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?
musicar northwest portland or musicarnw.com
Web site migration complete, still repairing problems. Sorry!