my cousin had 6inchs on the rear parcel shelf that seems to be the limite for width, a 6x9 may fit but it would involve cutting a lot of metal unless like my idea you could use an extender to lift the speaker and have it sit in the 6inch hole. Only problem may be clearance with the window.
I was gonna put some speakers in the back but looking at the window they would be very vulnerable to leaves in a smash and grab situation and since the car is parked at a station i thought no.