The black and black/yellow wire on the ignition switch connect to the two "M" terminals on the switch. It may be that either one or both have become disconnected at the switch. When the key is turned to OFF, the two "M" terminals are connected thus connecting the ignition kill circuit to ground. If the wiring at the switch is ok, you need to check back at the engine and anywhere along the line for an open circuit. I find it highly unlikely, but it is possible the ignition switch is bad. You can check that with an ohm meter. Disconnect the two wires, turn the switch to OFF. Connect the meter to the two "M" terminals. If the switch is ok you should read continuity (closed circuit). If it measures open circuit the switch is bad.
