• Power Antenna (yellow wire) - has power when radio is switched on (regardless of ignition state)
• Relay connected between Pi and power source
• Monitor PA to determine state of relay; If power to PA, then close relay (Pi on), otherwise open relay (Pi off).
• Monitoring PA state will allow for a software shutdown to be triggered before opening the relay

So - need to investigate what to use to monitor the state of the power antenna!