You can try this way. I am no expert but I think it will work.
Car Ignition Wire -> Diode ----> Opus Ign Wire.
Batt+ or alway on wire -> Switch -> diode -----> Join to Opus Ign Wire above.
Normally the switch is OFF so that the Car Ign wire control the on/off of the opus as intended. If you want the opus keep operate without the key/ign, flip the switch to ON then turn off the key.
I have similar setup for my PSU.
car IGN --> Diode ---> Switch (for manual shutdown) --> Relay control on.
car ACC --> Diode ---> Join with switch wire above.
On my car, the ACC line cut off during cranking. The above wiring keep the computer running when the key in either ACC or IGN possition and during cranking.
BTW I use inverter and the relay is to control the inverter switch.