I have written hundreds on this. Maybe more. Voltage drop in the harness, terminals/ connectors and switch, and sometimes the ground circuit
To check, turn key to "run" engine stopped. Stab one probe into the top of the POS battery post and the other as close to the VR IGN terminal as you can get. Or connect to the "key" side of the ballast resistor. You are hoping for a very low reading, the lower the better. anything over about .3V (3/10 of 1 volt) you need to chase it down. Culprits are any connections, the bulkhead connector terminals for the big red and big black ammeter circuit, and the dark blue "Ign1" "run" line. Also the ignition switch connector and the switch contacts themselves
One way around that is to electrically cut the "IGN1" run wire where it comes into the engine bay, use the firewall end to trigger a relay, feed the relay off the starter relay "big stud" and run the engine bay end of the run wire to the switched contact.
GROUND: Make this test with engine running fast to simulate low/ medium cruise RPM, and first with all accessories off, and again with them on, heater, lights, etc
Stab one probe of you meter into the top of the NEG battery post, and the other hard into the mounting flange of the VR. Again, the lower, the better, zero is perfect