I had my dash and support out of my 79 SA and don't remember if this seal goes in there somewhere.

 

Looks like it goes with the rear bumper

 

Yep that's it... Thanks, I had that off too LOL

 

Yes, it goes on top of the rear bumper, between the bumper and the body. You'll need to disassemble the whole thing including the inner sub structure to mount it. its about the first thing to position when putting the whole thing together.