Don't need one. In this case it was just to illustrate current flowing.
A light can be useful when hunting a short to ground.
Since there is an ammeter in that line, it also shows whether current is flowing. So it does the same thing.
Essentially very little drop in voltage for an approximately 5 amp draw.
from this it would seem there's no problem from battery to main splice.
Assuming you had the parking lights on when you made that last probing. Then yes, it would seem that the problem is between the bulkhead connector and the main splice. Since the only things you were touching were the connector (or connectors?) it is likely something loose right at the connector. Such as either terminal (the metal part) slipping out of the connector when you put them together Or could be wires broken at the terminal's crimps.
