There isn't anything particularly special about the rear speakers either physically or electrically. I believe they are identical to the dash speakers with the exception of a little additional wiring to function correctly with the front tweeters. (Thanks to MugsBRZ for the images below.) I don't know why there isn't a direct replacement but it wouldn't be difficult to adapt something as there is ample room behind the panel. If you have access to a 3D printer you might consider these 6.5" adapters:
https://www.youmagine.com/designs/6-...racket-for-brz
Another opinion put forth on the forum is why mess with the rear speakers? Focus on the dash/door speakers and add a proper sub-woofer in the trunk.