I built custom metal brackets to mount speakers under the backrests of my rear seat in my '67.
There is enough room back there to mount them as long as you don't carry passengers alot.
It takes careful measuring to get them exactly between the struts that form the structure of the back seat, but when completed they face forward and sound better than any other alternative that I have seen and heard for a convertible.
I'd post pics but I just got it back together and don't want to take it apart. I should have taken pictures before I buttoned eveything back up.
I cut a hole in the plastic portion of the 1/4 panel just above the seat and behind the ash tray. I installed 6 1/2 " speakers there. They sound good when balanced with front speakers located in the kick panel.