well the first thing i would do is pull the dash and look for anything unplugged, the ammeter is hardwired into the harness so if the dash came unplugged it would still work without the others,
if it is plugged in i would check for a 5 volt signal on the poles of the guages on the back, it should see it rise and fall rise and fall as it is not a constant voltage, see what you find and let us know. good luck
jamie