Can you Install speaker 6×9 on the rear of the car...

 

I put 6 x 9 speakers in boxes in the rear under the cover. the cover won't support speakers for long before it will give way . You could build a replacement out of marine type plywood, that would last a while before failure.
Regards
Dave

 

The factory spots are for the 5 x 10, however yeah, you could make something work I am sure. Just be aware of the issue that
Smilin' stated.............