I had pair of 16 cm very quaility Hertz speakers in my closet. Since BMW car audio system is crap and I didn’t find proper space for speakers in door I installed it in floor.
First disconnect battery, because if you won’t your airbag light could turn on on dash.
You need to remove the seat and disconnect cable from seat. Then unscrew top cover. And then under a carpet you will see 2 m6 nuts. Unscrew them and take out box. It can be tricky.
Here is stock floor speaker:
Then I cut with saw suitable speaker adapter bracket from 19mm mdf board-plywood and painted in black so I looks better and it is protected from moisture.
And stock cover will fit, thats nice 🙂
Here is how it looks in the car.
I will also install amplifier and connect those speakers on it. Because headunit doesn’t provide enough power for the speakers. But I’m happy with install. Sound is much better now than with stock speakers.