The audio dealer installed it (upright) right behind the left rear seat in the trunk.
The screws got loose over time and I decided to move it and see how it would sound.
What I did is install it (horizontal). I secured it to the back of the left rear seat and to the floor of the trunk.
The difference in the sound is obvious. It sounds much better now. I'm not aware of any problems with mounting it laying flat.
Just thought I'd throw that out to anyone who has it mounted upright and might want to give this a try.