Looks like maybe the ammeter gauge? The wires to the ammeter should run into the harness that goes into the cab and should come from a splice near the area your wires are in. Do you have an ammeter, and is it working? Can you take a voltage reading on the wires (one at a time)?
Also check here, I don't see a diagram for the 71, but maybe 72 will be close enough? http://www.fordification.com/tech/schematics_h.htm