Since you have made your way to the rear following the ignition control voltage and you seem to be hearing the solenoids click and the voltage across the large terminals that you are within a few feet of the fault.
At some point you need to sharpen one end of the probe on the meter or use one of those plastic headed thumb tacks to pierce the cable beyond the solenoids.
What you really need is a schematic. Those solenoids you are testing may not have anything to do with the starting system or could be things like a grid heater control.
Since you can control the starting solenoid by adding your own wires you know that part of the circuit is ok. Again, seems you are but a few feet from the problem. It is near impossible to physically follow some wires. The use of a camera can help enlighten things with flash pictures in places you can't stretch your neck or body. No days a go pro on a stick would be a great tool.
