Aftermarket alarms are notorious for causing battery drains. I'm presuming you have done simple checks like making sure things like interior lights/glovebox light etc are all going out.

If you disconnect the positive lead off the battery then connect a multimeter (set to amps) between the positive terminal on the battery and the wiring clamp you should be getting a high reading. Start pulling each fuse in turn until you see the reading drop. This should pinpoint which circuit is giving you the drain, so you can go from there.