Car alarm - parasitic drain and ringing?

If I understand you properly, your battery drains dead with the alarm connected, but if not connected, the battery drains and dies. So it's obvious that the problem is the alarm. So the question is...how? Here you have to go back to the old Radio Shack electronics kit. (I'm showing my age lol) The alarm control unit uses the car battery. It sends voltage over wires that by using capacitors, diodes, etc creates an "easy path" and a "hard path" for electricity. When following the easy path(s) from the positive battery terminal to the negative, all is well. If you interrupt the path, the electricity has to go somewhere, so it goes to the siren. The siren usually has a backup battery in case the primary fails, but it doesn't run the alarm.

That's an oversimplification but if you want to learn more there's plenty of info already written you can search for.

So what can happen is if you have a wire or switch or something else partially grounding, it will constantly feed the ground with power from your car battery. Eventually it gets low enough that it can trigger what some car alarms have which is a tamper circuit using the second battery if power is removed.

To correct it, put your meter on and disconnect the various runs to the door sensors and such one at a time until you see the drain stop. Then you know what leg it is. From there you can follow the wire, visually inspect, and if you find a problem, use the meter to verify.

Hope this helps! Some useful links

https://hackaday.com/2020/03/10/solar-panel-keeps-car-battery-topped-off-through-obd-ii-port/

https://shoptoolreviews.com/automotive/diagnostic-testing/parasitic-battery-drain/34587/