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 :)