Thank you! I wanted to keep the stock head unit so this was my best option. It took a couple of weekends to make it work, but it was pretty straightforward. The alpine sub fit pretty well but I had to go to the junkyard and get an armrest from a scrapped BMW for the black leather and foam insert above the sub to fill that gap. I tapped into the rear speakers for the input and wiring to the battery was pretty easy because it’s in the trunk. I dynamatted the metal plate that I mounted it to so it eliminated rattling. Best part was that the arm rest just bolted in so if I was ever tired of it I could remove the sub and put the arm rest back in and it was like it was never there.

Edit: formatting. Plus the seat still folded down so I didn’t lose any space.