Anything connected at the battery will draw power through the battery feed/charge wires.
When the alternator is working the ammeter will show current flowing to the battery plus current flowing to whatever has been tied in at the battery.
That additional current is going through all those connections that were only intended for battery charging. This is the same issue as an electric winch tied to the battery. Consider doing a heavy duty "Fleet" alternator set up. My version is linked early in this thread. Where I have the MSD power, you can connect your new harness. It can even have its own fuse box etc, and you can use that for the power connection for a headlight relay.
What this does.
The ammeter then only shows battery charging and discharging. So it reads true.
The 16 gage fusible link continues to protect the 12 gage wiring from a battery short.
Current to any of the items connected at the new post comes direct from the alternator (assuming its running). This current no longer has to go through the 1/4" push-on type terminals in the bulkhead multiconnector.