Some thoughts:
1...Stop trying to check this using resistance. First, very low values of resistance are difficult to measure accurately, and current flowing through a bad connection can change the resistance of a bad connection.
2...."Think" in terms of "very basics.
A...The VR MUST be grounded to the battery. Sounds like you have that now, but CHECK it. To do this, run the car at an RPM to simulate medium cruise, say, 1500-2000 RPM or so. Do so with the engine warm and the battery "normalized"
Make this check first with all acesssories off, and again with heater, lights, whatever you have powered on
Stab one meter lead right into the top of the battery NEG post. Stab the other into the mounting flange of the VR. Be sure to get through paint, plating, chrome, or rust. You are hoping for a very low value, the lower the better and zero is perfect
B....The VR MUST have a GOOD connection to battery POS. Think of the pathway from the VR to the battery. If the wiring is factory, the path is something like this:
From VR "ign" terminal............to a splice in the engine harness to.........dark blue "run" wire..............through bulkhead connector............to ignition switch connector.............through switch.........out switch connector on the switch "bat" wire..................to under dash welded splice...........to black ammeter wire............to ammeter...........through ammeter..........ammeter RED wire.............through bulkhead connector.............fuse link..........starter relay stud..............to battery
EACH OF THOSE POINTS in the description is a terminal that can have a bad connection. Take the ignition switch, there is at least THREE places right there............The power going into the switch at the SWITCH CONNECTOR TERMINAL.............the switch CONTACTS.........back out the switch connector at the RUN terminal.
Right there you have a possibility of 3 bad connections. They can get worse with load and heat, and can change, AKA wiggling the key changes the contacts
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THAT is why I use voltage drop, because a few tenths here, and a few tenths there adds up. You can "rig" a long scrap of wire to the battery POS post (or starter relay) and trail it along to your meter, and "chase" voltage drop down the path.
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RELAY!!!!!
Sometimes, there are situations where "it is just easier" to install a relay. Electrically cut the blue "run" wire coming out of the bulkhead into the engine bay. Use the firewall end to fire a relay, and feed the relay contacts power through a fuse / breaker from the start relay stud. Power the ignition/ field/ VR off the relay output. This puts all that stuff VERY close to the battery POS and eliminates "lots" in the "path" above
