Has anybody ever replaced the rear speakers in a 1st gen convertible?

Not sure about 1st gen, but on 2nd gen you remove the quarter side panels and the speakers come out from the backside. The grilles have 4 studs in them, they go through the side panels from the face side. Then the speakers go onto the stud on the backside of the panel. Nuts thread onto the studs holding the speakers tight to the backside of the side panel.
Pretty much requires removal of the back seats to get the side panels off, comfortably. Certainly will need to remove the rear seat bottom. You can leave the rear seat back part attached, just be careful with the back edge of the side panel as to not break it. It goes in behind the seat back. I prefer taking the seat back out too. Just makes it easier for me.

I have done it a few times. Remove the door sill plates as well. Panel will lift up and off the car body. Take care with seatbelts retainers as those like to break.