Wires throught fire wall

Reading the wiring diagrams just takes practice. Put your finger on a line and follow it. If it reaches a junction with other wires, consider all those wires the same "point" in the circuit. They are all at the same voltage. These cars have very easy to read diagrams. Try reading one in a modern car, or even my 80's Mercedes. A table to the right lists the color and gage of each wire. A smaller gage number is a thicker wire.

The bulkhead connector is not as complicated as it looks. I recall your 1973 has 3 connectors. One is the engine harness, one is for lights and horn, and one is just for the windshield wiper. Find them in the diagram by the different bumps that "key" the plugs to the connector. The contacts get corroded and cause endless problems. Not hard to fix, but most people ignore it until constant problems, then they ditch the car.