Just connecting the 9v battery in parallel with the 12v supply would probably have the little 9v on the point of exploding very quickly!
You need at least a resistor to limit the charging current to the 9v battery.
I suggest that you use a 180 or 200 Ohm resistor in series with the connection from the12v to the 9v, to limit the charge current to about 20mA.
Or, a better idea would be to forget the 9v battery and to filter the 12v supply to the intercom in order to remove the alternator noise.
A nice big iron cored inductor in series with the supply from 12v and a capacitor to 0v would probably be the way to go.
JimB