I seem to lack basic understanding of unplugging a plug

[dmjlambert]

Hi
I am trying to troubleshoot wiring to the brake lights. I have this connector above the gas pedal on the firewall that goes to engine compartment and from there goes to the rear of the truck. It looks like a plastic connector going into a rubber cup. I can't figure out how to unplug it. Should this be as simple as pulling the plug? I can't get it to unplug. I would like to unplug it so I can isolate the cab/dash wiring from the wiring that goes from the engine compartment to the back of the truck.

1969 Chevrolet CST/10 Pickup

Thanks
David