So this isn't an original idea, at least for me. I actually came up with this back in 1995.
In 1995 I got my original SHO (1993, which is ironic because we got the 2013 in 2015, both white. Both no sun roof. Both built with in 10 days of exactly 20 years apart, both spent one year on the lot before original owner purchase.)
Anyway, back in 1993 model year, remote keyless entry (what we call key FOB now) was an option. My SHO did not have it, so I got the "dealer installed" kit ( I remember when A/C was a dealer installed option).
Most just wired it under the dash. Not me I wired it to the factory location in the truck. This involved running 9 wires the length of the car. BUT I put the horn/siren under the dash.
Everyone thought it was stupid. Because "no one will hear it"
Well even by 1995 car alarms were ignored. I didn't need the public to hear it, just the thief.
After install was done the shop decided to have a contest to see who could last the longest.
No one lasted more than 4 seconds inside with alarm going off. Outside it was barely noticeable.