Alarm Relay Using Ground When Armed Output

My 211HV has a ground when armed wire but no built in alarm siren/light features. How complex would it be to add the ability to honk the horn and flash the lights if armed and either the door is opened or the ignition is turned to start/run? Thinking a latching relay and a flasher hooked to the +12v horns and lights. The ignition and the interior dome light circuits would both provide +12v for the relay and the gwa is ground. I've never used a latching relay but in a normal bosch relay I'd see using those to energize the relay. That would cause the alarm to shut off when the door is closed or ignition shut back off and that would be more of a warning system. I want it to stay triggered until I disable the alarm and thus nullify the gwa. Downside of not using an alarm module is that this could run the battery down...