70 Dart vent door fell off ?? Fix?

I have owned this very solid 70 Dart now for 24 years. It's never lived outside since I've owned it and rarely driven. I went out to look inside it today for some reason. I noticed the passenger side lower vent box door was laying on the floor. WTF? I didn't have time to look at it. I just shook my head and closed it up and covered it back up. Is this common? Are the spot welded? Uggh.