Going by your pictures you will need to remove the rear glass and cut out the rusted metal and replace it. You will have to make the determination whether to fabricate a whole new panel or try and piece it out. That will depend on how much good metal there is to weld to. The removable part is called an upper back panel assembly. You can check with the Thunderbird vendors that sell used parts and see if they have a better one rather than repair yours. Try Carl Heller(partsetal), The Birds Nest or Bob's Bird House.