Having done auto security as a trade for a LONG time, I can tell you this - ANY security system you can have installed is designed to prevent the vehicle from being stolen, not its contents. (FYI - an "alarm" is simply a noise-maker . . . ) Don't let anyone tell you that any kind of sensor can prevent a thief from stealing contents. Keep in mind that no matter how elaborate your security system is, a tow truck can make very short work of it.
I like my odds having my vehicle equipped with the latest in security though - think of it like this, two cars in a crowded parking lot side by side - one has an alarm, the other does not. Which one do you think a thief is more likely to break into? Of course, this makes several assumptions:
>You didn't give a would be thief a reason to break into your car - like pulling into the mall parking lot with the top down playing Dr. Dre at 145dB for all to hear before walking in to watch a movie.
>You take precautionary measures NOT to leave your valuables in plain sight.
>IF you have a security system, the parts the would be thief CAN see (like the L.E.D.) are installed with some thought to their placement. A thief can VERY quickly size up the quality of the installation by looking at where and how the L.E.D. is installed. An L.E.D. mounted to the left of the steering wheel in a factory plastic knock-out is a GIVEN that the brain for the system is also to the left of the steering wheel - a SUPER easy spot for a thief to disable it. An L.E.D. flush-mounted at the top of each door panel just next to the lock plungers indicate a real effort has been put into the installation, making a quick defeat unlikely. Again, this is where you differentiate a security system from an alarm.
I had an '88 GT convertible for many years with a very well thought out (and extremely elaborate) security system. At no time did I ever have anyone tamper with the vehicle.
Do some research and find out who in your area specializes in auto "security" - pass on the guys offering the $99 whiz bang gizmo. Expect to pay five to six hundred dollars for a security system worth having - about half of this cost will be in labor. Insist that all connections are soldered, not done with T-Taps or scotch locks - to insure reliability and as few false alarms as possible. (Having your alarm go off every morning at 3 AM from the cat is a sure fire way to have the neighbors ignore the sound of it during an actual break-in.)
Do your homework, and you'll find exactly what you need to keep that 'vert in your possession for many years to come.