Trigger Relay Only from one specific grounding path?
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 &ndash; Schematic created using CircuitLab