using a relay to back up if siren is cut

Back in the 90's when I started in car Audio I was reading a magazine where a guy installed a security system for competition (secuity system competition was big then)

He installed some relays to sound a back-up siren / horn if the main siren was yanked out or the wires were cut.  I thought this was a pretty cool feature but can't get a grasp on how this might have been done.

I wouldn't mind doing something like this in my Firebird and hid a siren in the rear somewhere, only to come on if the main one was disabled.

Anybody have any ideas?  There has to be a way to do this?

Thanks!

Ok, I've thought about this,

I should be able to take a relay and supply either a positve or negative signal to it and when that signal is severed (ie; someone yanking the main siren out or cutting wires)..it cause the relay to supply a ground to the negative of the second sire and cause it to emit sound.

The only way I can see this working is to solder a 2nd wire (to either ground or positive) on the main siren up REALLY close to the base and the act of cutting the wires will also sever the "special" wire going to the 2nd siren and doing what I mentioned above.

I hope this makes sense

Just mount the siren against the firewall and and be creative in you wiring (i.e.within the factory car loom). Dont seem like its really worth the time and effort. If im able to get under the hood, Ill just undo the whole battery cable. Some people use a backup battery DEI 520.