Stereo would likely mean the Front speaker is one of the channels, and the Rear speaker the other channel, that's the way it is in my 70. I can't remember if Front is L or Right, etc.
And I think the rear speaker would be in the middle of the rear seat backrest.
This is what the unit looks like. I'm sorry I don't have a picture of it mounted. This fall I looked at a 67 Caprice parts car for sale that had one with the Strato Bench seat.
I never thought to look at the rear speaker covers but the front has a speaker in each kick panel with a small mesh metal grille over it.
Here's a list of ancillary parts for the Multiplex unit, plus pics of the various Delco Multiplex units plus the 8-Track switch.
The * is for the yet un-named Camaro. They printed up everything and added a 1/2 page on "What is a Camaro?" to try to preface the * that appears throughout.