is that porn on your monitor? i see legs.
i would say put it in the vents, but it looks too big. if you already have a siren outside, how about hiding that one on the inside. you know whats really cool, if the alarm have some kinda delay feature on the additional sirens.
and no, the alarm probably doesnt have the exact same colors matching your siren's, but just look for the siren wires. or something like that.