Strange Dust From Camry Air Vents

My daughter accused me of shaving in my car. Sure enough, when we investgated her evidence, the dash and other horizontal surfaces had a fine black dust that looked like what you would get if you emptied an electric razor. I cleaned it up but it came back in a few weeks. I checked the cabin air cleaner and it was fine. I was finally cleared with my daughter when the other family car turned up with the same dust, also a toyota.



Can anyone clue me into where to start before I rip off the entire dash?

I would start with the blower motor.