I know on my 68 there would have been no way to pull a wire from the AC to the breaker box from what I saw when I had the rear cap out when I did the new floor and bathroom.
I'd bet the end of the wire is in the wall by the box. I just went out and looked at my breaker box. There are 3 wires coming out of the wall on the left side of the box. Each has a gromment and they are space about 2 inches apart, stacked in a straight line. One for the curb side, one for street side and one for the AC. I'd get a hole saw and cut a hole just above the top wire by the box, being very careful not to go deep and nick a wire. Then you can see if the end of the romex for the AC is there. Maybe you could use one of the endo camera's that works with your smart phone that they sell on Ebay to look inside the wall. PM me if you need me to send you a picture of what my breaker box area looks like. If you only have 2 wires coming out of the wall and there is a wire in the celing at the AC location then it should be there. If it is there you may have to cut a bigger hole in order to actually get to the wire and bend it out to make the connection to the breaker box. You could make a patch panel with a small hole for the wire and rivet it back in place.
Bruce & Rachel
68 Trade Wind
2001 Toyota Tundra