The ammeter, when properly wired, shows whether the battery is charging or discharging and how much.
The battery is nominally a 12 Volt power supply.
The alternator is nominally a 14 volt power supply.
The two power supplies are joined to the sub circuits at a welded splice, not the fuse box.
You'll have to fill in the rest, but adding additional electric loads requires knowing roughly what the loads are (watts or amps) to be sure they don't exceed the supply or ground wiring.
Ammeter:
Pulsating Alternator Gauge
If you want to keep these accessories than consider a variation one one of the heavy duty or fleet wiring arrangements. You can then add an auxilery fuse box for the new circuits and relays (headlights is a useful one) to the terminal block on the other side (alternator side) of the ammeter.
like this one
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