The heater case halves are held together with steel spring clips. Expect them to be rusty as hell - and don't lose them or you are toast. All of the air seals on the control flaps will have decayed. I used a roll of self adhesive foam seal (2mm thick by 10mm wide) to make new seals. The main flap has a big vinyl cover on it that isn't necessary. Just run the foam tape on the edges that make contact with the heater case.

...and, you're welcome