Per the wiring diagram you should have battery voltage on the white wire at the ignition switch. When the key is in the Start position the White/Black wire should also have battery voltage on it.
Have you also measured voltage on the White wire when the key is in the Start position to see if you are still getting battery voltage there or it it too is dropping to low voltage? If it too is dropping there is likely a loose or corroded wire between the battery and the ignition switch.
The white/black wire also provides relay coil voltage to the starter disable relay. If that's the relay you are hearing click, then it seems unlikely that you're actually only getting 2 volts or so on the white/black wire. I doubt that would be sufficient to close the starter disable relay.
-Rod