Suitable power source in trunk for relays

Hi guys, at the moment I’m building a power amplifier in the trunk that only reaches via remote. So I want to switch a relay between my duration plus and the remote connection that is best controlled via an ignition plus. My question is: Are there already existing lines in the trunk that I can use as relay triggers? I know why I don’t take remote from my Sc805? Because I can’t tap the signal from the middle pin of the 6Dins connector on Cinch. do you think of the idea of taking the plus from the petrol pump? Or maybe from the rear lights? Then theoretically the power amplifier would only be on if I had light on.