I don't think that you have fried the ammeter. If you had, it would probably be "open" so no current would flow through it and the truck would not start.
Did you try the headlight test? That is watching the gauge while turning the headlights off and on? If the guage doesn't move, you can disconnect the three wires from the load terminal on the ammeter. Turn the headlights on; if they still light up then the wiring for them is bypassing the ammeter. If the headlights don't work, run a wire from the load terminal on the ammeter to the headlight switch. (one of the two terminals at the back of the switch with fat wires attached) Try the lights again - if the lights now come on and the ammeter doesn't move, then your ammeter is probably toast.
