Road Runner Nest

On my '71 GTX there's a wire coming out of the firewall with a rubber cover on it. It goes into the fuse box, and in the middle of it there is a female connector on it that you connect something onto it. It's not the reverse wire, that has two wires on it and it's there. The car is a four speed.
Any idea's? :

Where on the firewall is the plug located? If its on the left hand side up high by the master cylinder it could be the switch wiring for an air grabber setup.

I'll get a better anwer tonight at home with my 71 shop manual.