All the current to supply everything except the starter itself runs through the ammeter. So if you have a short anywhere else after the ammeter, then the ammeter and the two wires in and out of the ammeter are also electrically shorted. So for example, if the supply current to the brake switch or reverse light switch was shorted to ground, the ammeter is also shorted to ground..
The ‘hot’ items, those not switched somewhere are the reverse/stop lights, dome light and the horn. All other circuits have some sort of switch before the circuit is ‘hot’.
I’d just explore each circuit one by one until you find a short or determine that the circuit is ok.