I recently came into contact with a few of these. Basically, it replaces (or can you can add to) your current siren. If someone gets in the vehicle and cuts power from the battery or to your alarm, the siren will keep sounding because of the built in battery.
Installation is extremely easy. You have a ground, siren wire coming from the brain, and you hook it up to 12V. The wires do not have to be hidden, even if they cut the ground, 12V, and signal wire from the brain, the alarm will still go off.
It comes with 2 keys, the keys can be used to turn off the siren completely, test the siren, and activate the siren with the alarm. It is the standard 6-tone siren, but you can remove certain tones if you dont like them.
Here is the link:
Asking price is 35 shipped to the 48 states, will ship elsewhere, shipping will not be included.
If you have any questions you contact me through here, my email is email@example.com and my AIM ID is exalted512. I'm a car stereo/alarm installer so I can answer any questiosn you might have.