Some of you may know that I restore underdash harnesses to OEM functionality. While gathering up all of the wiring diagrams available, I've managed to obtain (by reverse engineering) the 1967 and 1968 tach harness schematics. There is no known public domain schematics for these critters.
In the course of restoring these old harnesses, I have found many bad molded plugs, particularly at the engine side of the firewall. The crimps within these molded plugs are just poorly made. I've also found that about 20% of the headlight, turn signal, brake switch, etc. pins are also poorly crimped, but at least one can re-crimp these puppies. The molded connectors cannot be fixed. Many of your problems may be due to these poor crimpings, leading to no continuity of circuitry.
I have many parts, connectors, pins, etc. that y'all may need in your restoration efforts. In addition, I can probably answer any of your electrical questions you may have. Feel free to use me as a resource, either here, or via e-mail at firstname.lastname@example.org
I'm at your disposal!