How to safely ground DRL PWM signal?

Hello,
I am looking for a way to safely ground the PWM signal that is generated from the Daytime Running Lights FET circuit to an external circuit I built which uses vehicle ground (batt negative) as a ground source.

I currently have a circuit built that works 100% correct on my test bench but acts differently when I connect it to my vehicle. My guess is that the DRL PWM signal in the vehicle actually floats which is causing my op amp circuit to not trigger at the desired point.

I was thinking of finding the FET circuit in my vehicle and tying one side of the signal to my circuit ground through a resistor so they are both referencing a common ground.

Alternately, I would have to tie my external circuit to the same reference as the DRL PWM source instead of using vehicle ground.

Any ideas?

where is the circuit?

You probably just need a pull-down resistor so that the OP-amp input is low when the FET that powers the DRL is off.

Many DRLs are LED, and there is probably nothing to pull the circuit below around 6 V. I suggest a 1k resistor to ground as a starting point.