Ammeter pegged...
OK. Plus indicates Charging.
That diagram is wrong and frankly I believe it confuses the issue. The ammeter/ alternator wiring does not go through two separate connector. But the way that is drawn makes things difficult to read.
The "gist" though is that all loads and the alternator are on ONE SIDE of the ammeter. ONLY the battery is on the remaining side.
Whichever is more positive is what supplies power. If the car is not running, or is running too slow to charge, the battery is at a higher positive voltage than the alternator. The way the load current flows, causes the ammeter to deflect negative
If the alternator is charging and is capable of keeping up with the load, AND IF the battery is low enough that it "wants" a charge, then the alternator is more positive, and this causes the ammeter to deflect positive
If the battery is completey charged, if there are no harness issues, and if the VR is working properly, then the alternator output at 14V "floats" the battery and the ammeter remains centered. If a load is turned on, the VR jumps the voltage to maintain the 14V and the ammeter remains centered.