Door Rain Gutter on rear of 69 Dart - Rust issues

You're not going to want to hear this, but what you should do is cut that roof off and weld a different one on.
JMO of course.:D
as far as what I see trailbeast is right. overall It is better to spend the extra money and buy a car that has a good body or if I saw the back window from top to bottom in the first place I would have told you to replace the whole thing. and if you did post it I am sorry I didn't see it. looks to me you need to replace the out side of the back window sure round