Lol I don't think that e30 with the front spoiler is moving to the crusher any time soon. Looks like it has been sitting there a looooooong long time.
Good finds anyway
Your AMP should power on via the REM wire (usually BLUE), which is a 12v wire from the headunit. So when the headunit turns on, it powers the amp on too.
If it Stays on, you probably haven't wired it up correctly..