Torn doors???

If your talking about a crack at the base of the rear window pillar I had the same crap on my 89 GT (no T tops just sunroof) ... was cruisin down the interstate and noticed as I was getting up to speed I could see daylight through my weatherstripping as the window frame flexed outward. Not sure how common or what causes this exactly tho sorry I can't help there but my car had the same prob when I first got it (has since been welded up and hasn't rebroken so far )

my 91 had the same problem. not sure why on your car, but on the doors with frames I'm asuming it's caused by people pushing on the frame with the windows down to close the doors and it's flexing? or maybe how the window track is fixed inside the doors and the metal being thin in this area gets flexed from the windows going up and down and over the years a crack forms.

I had to have it welded at a body shop. came out nice. they guy at the shop said he has seen it before on mustangs although more common on gms he said, like the mid 80's monte carlos , grand prix etc.