Trigger Relay Only from one specific grounding path?

Question

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).

^{simulate this circuit – Schematic created using CircuitLab}

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?

Answer 1

You will need to cut the grounding path between your light relay and WDI and insert a diode pointing towards the WDI, and finally connect your new relay minus terminal directly to the WDI pin. This will work assuming that your light relay can work with some 0.7V less coil voltage, use a schottky if you want to be extra sure, but I think that relay can reliably turn on off of 9V so that should not be a problem.

See this schematic for a little clarification.

^{simulate this circuit – Schematic created using CircuitLab}