Voltage Problem
Diagram illustrating measuring voltage loss between the relay junction and the bulkhead connector.
Voltage drop by comparing voltage to ground.
If access is difficult or you don't have a long lead, a good wire that does not have current flowing in it can be used as lead extension.
For example, if the alternator's output terminal is accessible, it connects to the main splice.
With either headlights on or hitting start, measureing as illustrated below shows losses between battery positve and the mainsplice.