You could have two of these "Water Proof Switches"
installed. I was thinking about doing the shaved door deal. Two of these can be easily installed in the rear fenderwell on the drivers side. Its a perfect hiding spot for these. If the alarm is set and someone happens to find the switches, the alarm will sound.

Also, If your car battery dies, here are two more options below.
the first option you can find here. and the second option, if you look around online, you can find a kit that lets you hook up a 9 volt battery to a switch for emergency power to the solinoids.