yes it's typically to put the brain under the dash but it has to be well hidden and impossible to find. im sure any professional installer will know how to hide it. so don't worry.
when i had my alarm installed today, i asked the installer where the brain and backup battery were, he said they were under the dash. then i asked him to rip out the lower dash and let me see, I took more than 10 mins to find them. so u can imagine how difficult will be for a thieve to find it. i have the main siren somewhere under the hood which i don't know where exactly and a self powered backup siren inside the vichel where i can easily to access for the key. since it's selfpowered, it is not 12v, but aroud 8v. when the alarm is set off, both sirens will sound, but one will be louder than the other. my installer told me, the backup battery will hold up at least for 40 mins, well even 10 mins is enough to get ppl's attention unless the car is parked underground. yes, a backup battery is well worth the money, cuz if u don't have it and assumed that thief disconnected the main battery, the alarm would be dead as fawk. i say get it, if u have money. hope this helps