Connecting both the red and black through the bulkhead connector reduces the current through each bulkhead contact by half compared to just using the red through the connector. Reduces the circuit resistance and risk of overheating the connector.
I meant using just one wire through the bulkhead, but passing the wire directly through the bulkhead, eliminating the factory bulkhead connectors. In that case, shouldn't one wire from the starter relay into the passenger compartment be plenty? Especially given that everything's coming through a 16 ga. fusible link anyway.
