Straight to the short version:

In addition to the rusted rear trunk, there are what appear to be 90 degree reinforcement brackets for the bumper mount that are also gone.

Are these small brackets available somewhere or will I have to do some fabrication ???

I'll try to get some pictures posted tomorrow.

Try AA. Or you might get someone to cut them off of a parts car for you. I have the same problem on one side. I plan to fab a new one.