I don’t want to give away all my secrets, but a good after market alarm system will have a signal wire when disarmed by the remote. Use this to drive a fuel cut off solenoid someone inline between the tank and engine. That way you don’t have to worry about activating a kill switch each time you drive the car. (When you unlock the car it deactivates the fuel kill).
Secondly, add in a real time GPS tracker so you can track the vehicle
The 12volt guy has some slick kill switch circuits that tie in with your radio antenna up/down wire.