12v power wire

I was hoping if somebody could point me in the right direction for attaching a 12v power wire for a small powered subwoofer box I'm putting under the rear seat. I already passed the wire through the firewall but I do not want to mess something up with the batteries if there is a special proceedure when attaching the power wire to the 12v battery. I've heard people do this successfully but I do not want to make a dumb mistake by treating it as an ICE vehicle. Thank everyone.