Go to main power and grounds that may be paired in crimp butt connectors test there where issue may be or at the light daisy chain of grounds. My issue was the thermostats on both I don't remember if it broke the ground wire or power wire. But pretty sure it was ground so try bypassing it , if its in the vent and has bulb , then it fails open . As I repaired thermostats and replaced both fan blades the daisy chain in crimp connectors became apparent with lights dropping out.
In case thermo here is link actual part # for FF order was cancelled from only place that showed stock. This one matches tag from OEM by the way they accepted $20ea offer for a pair.