If your alarm goes off so much, aren't you basically crying wolf anyway? I would just put a switch on the siren so that you can turn it off at night, but still let the alarm keep the engine from starting. You may also want to add a fuel pump cut-off switch in some odd place like the hatch to prevent the car from being started even if the crook figures out how to bypass the alarm.
------------------
351W 89 Mustang GT Convertible