hi I recently couldn't figure out where to find the location of the alarm system/ siren on skoda octavia II (MK2) (1Z3) 2005 but now i can help you 

showing you step by step how i did it 

 

Here’s a link showing you how to do it  but it took me time to figure it out.     https://workshop-manuals.com/skoda/octavia-mk2/vehicle_electrics/electrical_system/lights_lamps_switches_inside_anti-theft_system/anti-theft_warning_system/removing_and_installing_the_hornh12/

 

           How to remove anti-theft siren step by step 

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*Required tools

-T20 screwdriver

-6-8mm  Iron Drill

-13mm wrench

-10mm wrench

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–	Switch off the ignition and all electrical components. The vehicle must not be locked via the radio control receiver of the central locking.

- I also disconnected the battery

–	Remove the front right wheelhouse liner (passenger side)

 

 

 

You do not need to drill out anything, leave that in situ and you can bend the box it sits in back to access, remove and replace alarm and then push back into place.