That is a good reprensentation 70's style aftermarket radio equipment of how to install rear speaker's in a coupe, wagon or hatchback with low flush mount grille's that would take a lot of abuse.
and you have a also on the coupes the rear area under the back glass and between The seat back.
On the hatch backs I have seen those Intusive wedge packs style mounted everywhere.
some really small "newer design" speaker coud be installed in the
little cover for the jack and tire tool at the rear lamps,,, but my plastic cover is losing the grille bars and is realy britlle just a roumd the grille like openings and I was
thinking of adding a temporary or set of covers much like the flush mount style in the picture's above ,also that is where the most of the insulation in the body at the foward rear wheel well's is thickest from the factory,Mike