I want to hook up an aftermarket siren to my factory alarm..

I already have found the ways to mod a shock and glass break sensor onto it, anybody have any ideas on how to get the siren to mod on?

1996 Nissan Maxima GLE
i thought about tapping into my HORN fuse at the fuse box for the sirens because the horn goes off when the alarm is triggered, but then i thought that the horn lead probably pulsates and i probably need a constant feed for a siren.... this is for interior sirens probably BTW