I'm trying to keep this simple M, intentionally ignoring some terminal connections to paint a clear picture, you want to do voltage test under load, switches activated, not wires disconnected, when you're looking for voltage drop. What you basically have is power going from positive battery terminal to circuit breaker (again, I'm leaving out some terminals). Test voltage (under ignition load) at battery terminal, good voltage, move to circuit breaker. If voltage good at breaker, move to power input terminal of e-stop. If voltage good, move to power output terminal of e-stop. If good, move to input of selector contacts. If good, move to output terminal of selector contact that is switched on. If voltage drops at that terminal, you have a poor conductor inside that contact block causing voltage drop.