Either your meter is AFU or you have some problems. This sounds like a voltage drop problem in the harness IF YOUR meter is accurate
The problem these old girls face is the connections and wiring from the battery to the resistor, so you have the fuse link, the bulkhead connector (a couple of times) the ammeter circuit, the connector in and out of the ignition switch and the switch itself.
So turn the key to "run" engine off. Stick one probe right on the battery positive, the other probe on the dark blue coming from the key to the resistor, IE at the key side of the resistor. You are looking for a low voltage, the lower the better. 1/2 volt is generous. If it's more than .2 v, consider doing some checking. If it's more than 1/2 volt DO do some checking.
Also, measure the battery voltage to ground. Should be above 12V.
Your main suspects are the bulkhead connector, the connector on the ignition switch, the switch, and the push on connectors on the ballast.