Finally installed my alarm today!
You could install a switch against on of the hinges as a trigger.
I've got a (cheap) alarm in my Dart aswell, but mainly for the insurance reasons AND the key-less entry.
There's nothing more annoying then having to fumble around with keys, especially in pouring rain or freezing cold for instance.
Only thing I didn't connect was the immobilizer-wire(s).
40 year old car wiring can be scary enough already, last thing I want is some cheap Chinese device trowing more misery into the mix :)