Here is how the power flows from the ignition switch to the starter relay, and then from the relay to through the park/neutral switch, and then from the relay to the starter. You can test the power with a volt meter or a 1 volt test light.

If you have power to the yellow wire from the ignition switch when the key is held in the crank position, and no power to the yellow wire at any other time, the ignition switch is working.

From the switch, the yellow wire goes through the bulkhead connector. If you have power to the yellow wire after the bulk head connector, the bulkhead connector is good.

After the bulk head connector, the yellow wire connects to the starter relay. If you have power to the yellow wire at the starter relay, everything up to that point is good.

For the starter relay to work, power to the small brown wire on the relay has to pass through the park/neutral switch on the trans. After the switch trans, the wire through the park/neutral switch goes to ground to complete the circuit. If the power can't pass through the park/neutral switch, due to the transmission not being in park or neutral, or because of a defective switch, the power can't reach ground and the starter relay won't power the starter. If you ground the wire that goes through the park/neutral switch with a test light, you will complete the circuit, the starter may function. If the starter functions when the wire to the park/neutral switch is grounded, suspect a defective switch, or poor wiring, or a poor ground after the switch (assuming the trans is in park or neutral).

If the relay is powered through the park neutral switch, the small wire going to the starter will be powered with battery voltage when the relay functions. If the wire going to the starter is powered when the ignition switch is in the crank position, everything up to and through the relay is working.

If the wire to the starter has power but the starter does not run, The problem may be the wire to the starter itself, or the starter itself. I have seen the two prong terminal on the starter go bad inside. It looks good, but does not pass power through it.

If the motor will turn when the starter relay is jumped (or bypassed), the wires to the starter are OK.

The system is not complex. You simply follow the power supply through the system. When you reach a point the power in no longer present, you look for the problem between where the power last was to where it ended. Gene