I have a car wireless door lock system I want to modify. The basic operation is that when the key fob's button is pressed, a light relay is triggered, to flash some of the car's lights, to indicate the doors are locking. It does this by grounding a pin attached to the relay. This same relay is used on multiple systems.
I want to add a second relay, that only triggers, when the wireless door lock system triggers the light relay, not when anything else does. General schematic attached. (Red wires and New relay is what I know I'll be adding).
Any of the nodes labeled "Switch" "Alarm" or "Misc" can ground the relay, turning on the lights. If I simply tap into the circuit as shown, that would make the new relay trigger as well, and I don't want a 24/7 horn blasting.
Obviously, I only want the New relay to trigger, when the WDI grounds the Existing relay. How can I do this?