That should read "Could start a fire if...". On your original harness a wire ran from the hot post on the starter to the ammeter. From there another wire ran to the voltage regulator. Both had to pass through the firewall.
When Detroit went to volt meters they claimed it was safer because now you don't have the entire load of the electric system passing through the firewall twice. Less chance of fires. I think they did it because it was cheaper and pressed the safety angle to fool us into thinking they are thinking about our safety.
Your aftermarket harness probably has the wire you need to tap into out of the cab. It can be done, just be sure you insulate the holes where you bring it in, or it could start a fire.
