Wiring issue i suspect. The alarm control unit must be getting its constant power but might be missing its ignition signal or even the door switch signal too (very suss if you see 2 wires from the control unit being twisted into on power wire from the car). just a quick check to be carried out and see if there is a valid ignition signal to the alarm. While you are under there, see what brand it is and see if wiring schematics are available. GL