I don't have a picture that shows anything of the back speakers I put in my Mach...…….. and that was the intent, you can't see them. As these cars never had rear speakers, I wanted to keep them hidden. My car has a standard rear seat by the way.
Here's what I did; I bought a nice pair of 4"diam speakers. They will fit right into the steel frame. I bought a 4' x8' sheet of 1/8th" hardboard (had to buy a full sheet). I took the old package tray that some moron had fitted 6x9's in that looked like shit as a template and cut a couple of new blanks. Measured the hole locations and cut the two holes to the diameter of the cones, i.e. smaller than the OD of the speakers. Then I made up a frame out of 1/2" ply and screwed them to the board. That way there is more thickness to screw the speakers in to. This 'frame' will still fit into the steel without cutting it.
The material I found to cover it was sold as "generic automotive carpet" but actually is more like a thick felt like material and is almost see-though and perfect for letting sound through. The result is invisible rear speakers that have all the sound I want and the package tray looks somewhat stock to the casual viewer.
You will still need a double stereo speaker in the dash. NPD sell them as do others.
Just a suggestion if you don't want speakers showing.
Geoff.