05 Prius Anti theft Alarm ECU installation

Hello guys,

So, I recently bought a prius (05), and in the listing said it had alarm. I didn't even check if it had, and it's been a few months now and I realized it doesn't.

After searching a while, I found a video in which suggested connecting Toyota Techstream in order to activate the alarm (Since many base model toyota's share the computer with higher end models, and the settings are just turned off). However, techstream was all blank in the alarms section, which lead me to think there was no alarm installed. I also looked for a button to activate/deactivate alarm near the smart key sytem enable/disable button. There was none.

That being said, I searched online for Alarm ECUs and I thought about buying one used and connecting into my prius' computer, then enabling it in toyota techstream to sound by the horn.

While I wish it would work, I have some doubts regarding "sensors" that would trigger the alarm. I understand I won't have those ultrasonic sensors (for when hands enter the vehicle when its locked and has a window opened for example) unless I install them myself, but I was wondering if anyone had tried it before and, if so, how did it work.

I would also accept suggestions on how have an alarm installed without having to pay the hundreds they ask in my country.

Thank you all in advance.