There is ONLY ONE wire that is hot with key in "run" under the hood, and that is the "traditionally" dark blue "ignition run" wire (IGN1) as noted above.
BE CAREFUL AND CHECK IT:
1....It is NOT FUSED and you can " burn stuff down"
2....It is VERY commonly subject of voltage drop, causing low voltage to the VR and causing the charging system to run "high."
To check this, put the key in "run" with engine stopped. Put one lead of your meter on the blue resistor terminal, or the blue alternator field. Put the other probe on the battery POS terminal
You are hoping for a very low reading, the lower the better, and over .3V (3/10 of one volt) is time to start checking
Your suspects are "in the path" from battery to the blue wire, fuse link to the bulkhead connector terminal, the ammeter, the welded splice in the harness, the ignition switch connector, the ignition switch itself, and the "ignition run" terminal feeding back out through the bulkhead
Any one or combination of "some" of these can "add up" to considerable drop
ONE FIX is to break the blue (ignition run) coming out of the bulkhead. Use the bulkhead end to trigger a relay, and the remaining blue as the load end. I would put a 20A breaker in, and keep the wiring well protected. You don't want that to fail
