I think that you have already experimented and shown that grounding the parking light wire for more than a few seconds flashes the headlights, so your scheme should work. Of course, the horn honk set to constant will overide the parking light flash so you might not get that, but you probably already know how it will act. You don't have to worry about too much current draw since you are only activating a relay and not running the headlights off the alarm circuit.