I actually have the harness for that you are looking for, but I'd like to sell it with the whole setup. (rear deck as you have pictured), all front speakers, and the harness that runs the length of the car. For the rear deck piece.
I just removed it all a few weeks ago.
Those 2 holes by each 6x8 (in your pic) should have a threaded stud mounted in them. There are corresponding holes in the rear deck of the car. And the whole unit attached with 6 nuts total. The other 2 studs are out by the tweeter/mid speaker. You can see the studs in the pic below.
View attachment 17186