How to replace rotted boards?

In my targa, the vinyl is in excellent shape, except for the board giving it its form. I'm talking about the rear seats. How should I replace the board giving the vinyl its shape? Also, there is something that looks like it deflects heat, painted silver and has a carbon look. It has several cracks and two screws are missing, where can I find the screws, and what is this made out of so I can repair it? I'm thinking the home depot has most of this stuff, maybe I can get a realy thin piece of wood for the seats cushins. There is also a plastic trim that goes around the rear window, but it looks like heat realy deformed it. Should I try to iron this back into shape? Or replace it with something else?