Car Alarm System that can be wired into harness

[speedslave]

I am looking for info on the options , if any, of a aftermarket care alarm that can be wired into existing harness connecter and function in the same way as the factory system did. I'm ok with removing the existing security control module and possibly pulling the connector off of it and solder wires to it from the new AF system. Just wondering if there is anything out there that will work. The oem one is 30 years old and I'm on my second used one. So looking to see if anyone else has done this and what system was used. Thanks.