I'm assuming rear mount battery. If it doesn't need a legal, functional master switch, I would want to keep it short and run it to the closest terminal stud for the main battery cable - starter relay, junction block, etc. Not the starter to avoid clutter & hot pipes, not all the way to the trunk to avoid that long run (voltage drop). If it has a master switch that you want to work correctly - run it to the battery side stud on the switch or the battery itself and consider going up a size because of the distance.