Your ammeter is connected between the battery and the rest of the truck electrical systems. Apart from the starter and the horn, all loads go through the ammeter. If you connect a switched load directly to the battery with a fuse as suggested, that load will show up as a current in the charge direction. In other words you can end up with a discharging system and not even know it. If you need a supply you should tap off the load side of the ammeter.
