Folks used to do this "back in the day" when the little ceramic "sonalert" and other inexpensive noise / beeper modules began to appear, import, inexpensive. Just wire the thing with an isolation diode to the key "run" line and to the park/ tail light circuit. The diode will prevent the ignition power from activating the alarm, and when the key is off, the loads on the run line act like a ground for the beeper with power coming from the lighting circuit. Basically, tail light power is what you want as it is no "no matter what" unless switch is off