power feed needed

Having had my cat stolen recently I'm thinking of fitting some sort of alarm and need a power feed nearby (hopefully) but the feed would need to only be live when the ignition is off, is there anything suitable? Could I take a feed from the alarm system (volumetric sensor?) which is only live when the alarm is armed, my concern is I don't want to do something that will cause the normal alarm to wobble.
My second solution would be a permanent live with an ignition switched relay to cut the feed but that is a more complicated solution, has anyone got a simple solution.
I'm aware a cat bypass would solve the problem but don't really regard this as a solution.
Thanks

101% of alarms are more trouble than they're worth.
No one pays any attention when the damned things go off anyway!
My auto-electrician mate hates aftermarket alarms, whenever he encounters weird diagnostic problems on a car equipped with an aftermarket security system, the first thing he does is disconnect the bloody thing... most of the time the problem goes away, then he has to redo the amateur hour wiring efforts used to fit it.
Crap crimp connections and those bloody awful pigtail connectors are usually the problem (even with so called professionally fitted units)
My mate solders the terminals on and uses heat shrink to insulate.
This will make the system and car much more reliable, but wont compensate for poor quality manufacturing practices or mismatched components used in the alarm system itself, false alarms may still and probably will occur, as most, if not all of this stuff is Chinese rubbish.

Simple. The standard alarm works by sensing a change in current flowing through the alarm ECU when a door is opened and the interior lights go on. Just run a feed from one of the footwell lights through a switch connected to the cat.