Wiring query - Piezo alarm

[Ammonite]

I want to connect an additional (extra loud) 12v piezo buzzer to a Vetus exhaust alarm as well my bilge pump. I intend taking the positive feed for the buzzer from the pump as this is always live whereas the Vetus alarm is ignition dependent but I don't, for example, want the pump to operate when the Vetus alarm triggers, which would happen if I link both switchable negatives to the piezo. Would a diode in one of the negatives to the piezo suffice? Thanks

 

[andsarkit]

If I understand you correctly I think you will need a diode in each connection to stop the exhaust alarm sounding when the pump operates.
This is what I think you have but it may be wrong.

 

[Ammonite]

Thank you ever so much for doing this. From my (limited) understanding that looks spot on.