I've mounted rear speakers in the wagons I've had.

I used the pre-threaded mount points located just under the headliner, above the rear seatbelt posts. These are there so you can mount the Optional rear Luggage/dog cage in the rear cargo space. They are just above the lip.

You need an enclosure, I got ones that hold a 6" round from Crutchfield.

All you need to do is make a small slit in the headliner...1/2- which is hidden by the speaker housing.