theft insurance.

hate to say it...you can use all the theft prevention methods you want, but if they want your car bad enough, they're taking it. no way around it.

transponders are great for keeping someone from hotwiring your car, but a tow truck doesn't need your car to be running

diehard (i think) makes a battery that has a remote power cutoff built right into it. basically what drumsy was talkin about...it has a circuit breaker that allows power to flow to the radio and clock even when the cutoff is active, to prevent loss of radio stations, radio security code, etc.

if someone tries to start the car, the power draw is too much for the circuit breaker, and the power is temporarily 100% off. when you return, you deactivate the cutoff, and the car runs normally.

obviously breaking in, opening the hood, and putting a different battery in would bypass the system...but it makes the theft a little more difficult, and might cause them to move along to easier targets.

a really good method is using an alarm that has a wireless relay system. this allows a small receiver to be installed somewhere on the fuel pump power line, or similar vital system, to be activated/deactivated by the alarm brain but doesn't have any traceable wires to follow. if you have an ignition kill relay, someone can find the alarm brain and follow the wires to the starter kill, and bypass it. the wireless approach requires the alarm brain to send a coded signal to the relay that could be ANYWHERE in the car, and even if they find the alarm brain, there is no way to force that unlock signal to be sent w/out having the remote to the alarm. this is very very hard to bypass.

but like i said...a skilled tow truck driver can have a FWD car on a hook and rolling in a couple minutes...and nobody looks twice at a car being towed even if the alarm is going off.