installing security systems

[rudder]

I recently had my '96 explorer broken into and my entire stereo was stolen, so I went to circuit city and had them install a security system. However, I also bought a battery operated siren and a peizo siren, which I intend to install myself, since i suspect one of the employees that works there that i know may have had something to do with my stuff getting jacked. The problem is that I need to know how the hell I can connect those things in such a way that even a person who knows how security systems work cant break in. I need to know which wires in my dash do what, and also if someone snips the cord to my battery, wouldn't that shut off the brain even if i have a battery backup siren?