I have a backup battery connected to the alarm, and I want to run 3 sirens in my install. I understand you'll need a relay to power the 3 sirens, but if the relay is powered by the car and not the backup battery because the backup battery is specifically only able to power the brain, then when the thief disconnects the battery, it kills all 3 sirens.

Would it be possible to splice the siren wire to 2 sirens and splice a third wire to the relay to power siren number 3? So that way when the thief disconnects the battery, it'll kill the siren powered by the relay(siren #3), but the 2 sirens connected to the alarm would still be powered by the backup battery.

All input on what I should do is greatly appreciated.

 

Or run 2 back up batteries, I would use the relay to kill the other two sirens and let the back up battery power one siren thats hidden in the car and one that in the fender or hidden in the engine bay were it can not be tampered with.