There is an internal 100 ohm resistor inside the ignition switch which is used as an anti-theft feature (to avoid simple bypass).
When key-switch is turned on, one pole of the ignition switch connects the 12V battery power from the red/black wire on the ignition harness via the 100ohm resistor to an output on the Orange/Red wire.
That orange/red wire feeds into the ECM to provide the signal to enable the operation.
Not to be overly complex, but there is an additional resistor inisde the ECM that (along with the 100 ohm in the ignition) forms a resistor divider: so the voltage on the orange/red should be approximately 6.5V or so.

To troubleshoot:
1. Disconnect the battery
2.Unplug the ECM connectors & find the orange/red wire; use your multimeter on ohms to check for continuity between the terminal on the ECM connector and the opposite end at the ignition connector plug (Main harness side). This should be near zero ohms of course. (If open, look for broken wire)
Also check between the orange/red and the battery -ve cable - this should read open circuit. (If short, then look for trapped wire)
3. Connect your test leads to measure resistance between the red and orange/yellow wires of the harness going to the key-switch - this should be 'open' with switch off and 100 ohms with switch on. If NOT 100 ohms, then you have bad ignition switch.
4. If you do read 100 ohms, reconnect everything including the battery: set test meter to now read volts instead of ohms; read between the orange/red (orange/yellow on switch side) and the battery -ve terminal - you should measure ~ 6.5V.