I had the same prob on my e36 I brought in my system from my old prelude 2 alpine type S 12's which set off alarms in the honda. But did almost nothing in the Bimmer. Then I found that little cutout panel.
oh Might be wrong forum but I have an Alpine CDA-7897 Head Unit that has a shorted DC-DC converter if anyone wants to try and fix it. Its a $600 deck, and I'm not askin much if anything PM me.