[HIGHWAY BY THE SEA]

I recently bought a used wiring harness for the inoperable rear lights in my 67 3500 one-ton to replace the shoddy wiring that did not work anyway. The existing firewall male plug has three blades three wires and a slit for a fourth (backup light with light green wire) The female plug has four wires. I CANNOT seem to get any power to the rear lights from this setup. I checked the wires for continuity and they work. Do the front signal lights have to be installed and hooked up before the rear ones work? I do not have my front ones hooked up yet so I was not sure.