There is a section in the monster thread on installing that particular radio here :
https://xoutpost.com/bmw-sav-forums/...hread-140.html
That covers what wire to hook up to force it on. But the easiest way was coding the car to not having a dsp amp best I remember. This caused it to turn on and stay on. If it is set to having a dsp in the coding it will turn the amp off after powering it on when it cant communicate with the headunit you removed by installing the Seicane. That is probably the correct fix for this.