Why not just have a scr trigger from the interior light? The scr will switch the horn relay to ground. Door opens, light switch fires SCR, horn blows until someone turns off the hidden (under dashboard or seat) switch, which breaks the circuit to SCR. You could use a capacitor to give you a few seconds to leave or enter the car. After thinking about it, you may have to use a transistor to invert the signal from the door switch. You could set up your delay with the capacitor, resistor, to wait 10 or so seconds before firing the transistor, which would then set the scr. Back in the 70's when the price of gas was rocketing, people stealing gas from your car at night became a common occurance. I thought about attaching a small magnetic reed switch to the gas cap cover that would do the same thing. Just never got around to doing it. One guy I know wired a 120 volt hot wire to the frame of his car. I wouldnt recommend doing this though, it could kill someone and give you a manslaughter charge. (I know, it shouldnt be against the law to kill a thief, but it is.)