I've built a little Arduino circuit which controls a horn (high current), and I want to put it into my car (2003 e39), but I only want to power it when the car is on. I would prefer to put the whole circuit in the trunk, but I can be flexible if I need to be. I was hoping I could tie into an unused fuse, but after testing a few, it seems the fuses are all powered even when the car is off. Is there another place I could tie into fairly easily? Thanks in advance!