I did a little bit of research on the rare 1968 Mustang fastback (2+2) rear window defogger. Refer to the attached document.
I was unable to identify the part number for the interior grille that is located above the trap door. If you can help out with this please do so.
Note that I don't have the 1968 Osborn interior assembly manuals. If there is additional information from this source please advise.
And of course, if any of the information that I included in the document is unclear or incorrect, let me know that and I'll update accordingly.
Revision 1 - added at the end some sheet metal information from the MPC
Revision 2 - corrected and clarified based on post #2
Revision 3 - additional pictures added at the end of the document
Thanks, John