Hey Brian, the problem with an ammeter is that in order to measure current it has to be in series, or in line, with everything that's being measured. This means that the electrons it's measuring have to flow through a suitably large wire that runs all the way to the instrument panel before going to the loads they're destined for...so the wiring tends to be relatively large, heavy, and inefficient. This is a lot of metal for not very much information--instrument ammeters are not very precise. All you ultimately can tell is whether the alternator is working or not, and whether there is a small or a large load on the battery.
You can get the same information out of a simple voltmeter, which uses very small wires, plus more information about the current health of the battery(ies). I actually have two voltmeters installed on my boat, one in the cockpit that shows Volts, and one in the cabin which shows percent of charge.
I'd suggest looking carefully at the ammeter wiring and seeing if it can't be removed entirely. You may have to reroute some of the components, but I was able to pull mine out completely in about half an hour. It's possible that a previous owner re-ran the wiring without pulling out the ammeter, which would go a long way towards explaining why yours isn't moving.