This may be tricky depending on the circuitry of the car. I would have to have a look into how it is wired up before I would be able to do this. There are a couple of methods of doing it. You would have to set it up so that the horn only sounded when the car was being locked/unlocked, so you cant just tap a relay off the indicators or as accentstencil said it would go off everytime you used your indicator. I would imagine that you would need to tap off a wire coming from the ECU (cars computer) that was used for the lock and unlock signal. When the unlock signal went through the wire it would turn on the relay and the horn would beep. This is assuming the signal is enough to turn the relay on, otherwise you would have to go deeper with the electronics.