Flickering or pulstating Headlights

If there is no current to speak of through the ignition wire, then the resistance could be in the bulkhead feed or the igntion switch or connectors.
Think of it like the water pipes in your house. When there is no flow, the pressure is the same everywhere. It doesn't matter if the line to the 2nd floor bathroom is plugged up with mineral deposits. Only when the shower is running does the constriction cause the pressure to drop.