It should be bone dry back there unless you drive into a lake.
You are limited by a plastic duct on the passenger side when choosing the magnet size of the speaker. I machined a circular rim that brings my rear speakers forward a half inch. Putting speakers with a high frequency response back there is a total waste of effort. They fire into the back of the seats.

Putting dedicated woofers back there with a crossover will give you a buzz. Have fun.
"Round is good". Long travel is better.