Hi-
Here is what I would do:
If all the wires are black, the first thing I would do is cut some tape to wrap around each wire and have a marker ready to write down the wires function. If you have a test with with the aligator clip on one side and the pointer that lights the bulb in the tester when current is present "that would help here". First press the brake pedal down and with it down, probe each wire in the Column connector to find that hot wire coming from the brakes. When you found it, let go of the pedal and make sure it goes out to make sure that is the brake wire. Mark the tape on that wire "Brake". Next probe the wires with the tester to find the wire that has current with the key off. That wire comes from the hazard flasher. Once you find it, mark the wire "Hazard". Now turn the key on and probe the remaining wires for the wire with current with the key on......that is your current from the signal switch...mark that wire "signal". Now you should have 5 wires left..."left front signal, right front signal, left rear stop/signal, right rear stop/signal and the horn wire. With the brakes applied you should get current coming out of (2) of the remaining unmarked wires.."these go to the back stop lights. If you have no current coming out then you have a problem with the signal unit in the column. If you have current coming out of both wires and nothing to the back of the truck then you need to start tracing those wires. On the 65, their is a connector on the firewall "a 3 prong connector" and if it's not making a good connection you will not send any current to the back lights. Trace those rear light wires from the back of the truck, under the cab and see if they come up and into that connector.
