Electrical plugs on firewall...

[soupcan]

Hello All--
I'm trying to track down a short in the electrical system of my '72 truck. I was looking at what appear to be plugs where the wires go through the firewall near the power brake booster. I tugged on them a little but they didn't budge. Are these really plugs? Do they unplug from outside or inside the cab or both?? I've heard that you guys replace entire cabs, so I assume that something must disconnect somewhere!!

Thanks in advance...