Nacho. Yes I see. I'm not sure.
Lets assume the hanging wires taped together are the pair through a grommet. Then you are correct.

In the next photo there is only one wire on the ammeter. That seems to be the original to the main splice.


So, the best connection will be to remove the nuts on the ammeter posts and attach the hanging wires. If the ammeter is OK, it can be put into the circuit.
Then the only question is what happened to the original alternator output wire? If the connection is OK, like you write, it can be left in parallel. If not, it should removed such that there is no hot wire floating around. Same as done on the battery side.