Maybe this makes it clearer.
The original red (battery) wire from the bulkhead to the ammeter can be removed.
The original alternator connection from main splice to ammeter can be removed and taped off;
Or can be removed and connected as the new feed through a grommet. That might be a better connection than the bulkhead connector.
I say might, because some years and models and options had better bulhead connectors for the main feeds. I don't know what your truck has.