Voltage drop at ballast resistor

Totaly agree.

My point was understanding that opening a circuit means there is no flow in the circuit. Its possible to narrow where the resistance is in a circuit, but only if current is flowing. So with the bulkhead to the battery disconnected, the reading was battery voltage - but could have a just one strand of wire left. Connecting it, and turning something on verified that part of the wiring was OK.

So that's how I got to checking the three cavities in the bulkhead to narrow where the voltage is dropping.