A thought. I believe the way GM works is by using the ECM to ground the trigger on the relay.
What that means is that the Hot side of the trigger is always live, and the feed side of the relay is always live.
If this is correct, I would be curious to see if the issue happens when the relay trigger is grounded (engaging the fans), if it does it then, Unplug the fans, etc. That should help you trace down what part of the circuit is causing the issue if it is in fact back feeding.