There are several reasons for the connectors in the system.
First, it makes the production line assembly process possible. An end-to-end one piece wiring harness would require hours to install.
A multi-segment harness allows the individual pieces to be replaced if damaged, like in an engine fire or collision.
All of those harness connectors allow a mechanic to isolate sub systems & paths for diagnosis and testing. A bulkhead connector can be pulled, and tested for shorts & opens without testing unknown paths behind the firewall. The connector can be tested as well. Let's say you have no power on the parking lamp circuit at the front. You can pull the bulkhead, and test the circuit forward to the lamps with a meter. If it is open, then the problem is forward of the firewall. You can check for voltage on the correct pin at the bulkhead - if no voltage the problem is under the dash...
The biggest problem with connectors from the 60's & 70's is the unsealed nature of the connector. The wire enters the connector through a hole larger than the wire by quite a bit. Humidity, salt spray, smog, dust - all of it freely enters the connector. The copper connection corrodes slowly, even here in NV with 8% humidity. Pulling the bulkhead connector on my '65 & wire brushing the terminals helped a lot. Even pulling & re-installing the connector a few times helps clean the contacts. Be sure to blow out the junk with compressed air after cleaning.
Newer, more modern connectors have seals around the individual wires where they enter the connector, as well as seals where the connectors meet. They stay clean & dry when coupled.
That pin & sleeve weatherpack connector pictured in this thread was an early attempt at sealing. It is obsolete now, and the pin & sleeve design is not as reliable as the Metripack design. It is still much better than the 1962 Delco Packard design used in our early mopars.
The foam seal in the old style bulkhead does not protect the contacts at all. They are still open to the engine compartment via the wire itself. The foam is there to seal the passenger compartment from the engine bay fumes.
If anyone decides to wire a vehicle and delete the bulkhead, I would still recommend using some connectors in the harness. Be sure to use a proper strain relief to prevent chafing of the wires where they pass through the firewall.
Remember that term "Fire Wall" All openings should be secured so that smoke or fumes from the engine compartment can NOT enter the passenger compartment. A small exhaust leak can kill you with the wrong conditions.
Be safe.
B.
