After market alarms suck....In the Netherlands the insurance companies obligate to upgrade the factory alarm with after market crap (because they say that the original stuff is not good enough
All that after market crap works fine for a couple of years before it starts to take over control...
The siren can be placed under the hood wherever you want; you can mount it behind the front bumper near the horn as well. My after market siren is placed near the airco compressor (it works fine but the siren sound is turned OFF; it tended to haul in the middle of the night to keep everyone awake).
I have mounted a siren under the rear bumper of a 190E 2,3-16 once, because there was not enough space under the hood (besides the exhaust that went orange of heat). The car had 2 additional piezo sirens behind the console as well.
I have had a VW GOLF (my first car) with 2 piezo sirens in the ventilation units behind the grating; the sound was horrible.
We had a 186 300SL with the siren under the right seat (instead of under the hood). Sirens inside the car will help you (as the owner of a car) a lot more as a siren somewhere outside the car.