All I want is something that will blow the horn, and maybe flash the lights.
In it's simplest form, a switch mounted somewhere outside the car that turns the system 'on' by providing a ground path for a relay. When someone opens a door, the relay closes, current is passed to a flasher (like for your turn signals) and then to the horn and headlight relays. Very simple.
Also, this is instantaneous. As soon as they open the door, the horn starts. There is an 'always on' current source (courtesy lights) that do not turn off when the dome light is turned off, so I intend to use this for the 'trigger'.