I'd have to diagram it out to be sure, but it sounds like you have an ammeter gauge that uses a shunt. If that is the case, it is not a full current ammeter. Its just measuring the voltage difference between two points in the harness and calculating amperage.
My '75 GMC has a gauge wired like this, and it just uses a five amp fuse in each wire that connects to the gauge. They are inline fuses that are in the wire lines under the hood. The actual amperage flow to the gauge when you use a shunt is quite low.