This won’t help making it harder to steal but could scare the thieves off more - some of you may remember the old Chapman alarms that were available in the 1970s and 80s. They had an electro-mechanical lock switch mounted under the dash you pressed in and it engaged a steel hood lock, opened the fuel pump circuit, and had a high current air siren like this that activated via current draw from the interior lighting circuit
Both My parents mid-80s Regals had this. It was ear piercing when it went off. You could hear it for blocks.
I picked this one up a few years ago for dirt cheap on eBay just for nostalgia really but now I’m wondering if the siren could be integrated to the factory alarm in addition to the factory horn honk.
It runs on standard 12v dc. When connected straight to a battery you better close your ears.