Snowing Inside the mustang

Do you have a sunroof? My old 90 GT had a leak at the sunroof and would leak down to the front of the headliner and appeared to be the windshield. Either way, the headliner will probably have to come out to see exactly where it's coming from.
Hope this helps.

I doubt that snow is getting in through the windshield seal unless you've got a HUGE leak there. Check again. It's probably snow coming in the cowl and through the defroster, then swirling around near the headliner. You probably just didn't see it while it was going up the inside of the windshield since it blended against the snow outside. Once it got to the headliner it's more visible since it's white against another color.