This mod is adapted from my brother's work on 1Additcs with his 135i. My E90 M3 did not come with storage nets on the backs of the seats. They are easy to add. To install them, you must first remove the factory seatbacks.
The factory seatbacks are locked in with 2 upholstery clips that are one-use-only. They are located at the bottom of the seatbacks, shown here:
On a sedan, it is not possible to fold the seats forward so these are hard to get to. For this mod, move the seats all the way up and forward, and also move the headrests all the way up to their maximum up position.
I tried several methods for removing the 4 upholstery pins. I started with an upholstery tool shown here:
I tried drilling them out. In the end, what worked best for me was a woodworking chisel:
If you knock the head clean off, then the seatback is freed. To remove the seatback, pull it down a few inches, and then rotate it:
Once removed, you can see inside your seat as shown:
You will have to push the rest of the upholstery pins all the way up into the seat. They will not pull out, only in. Once inside the seat, reach in with your fingers and retrieve the broken off part of the upholstery clip so it doesn't rattle around. This is not easy. Installation of the new STOWback is reverse of removal. Once in place, install 2 new upholstery clips in each seat. You will need to tap them in with a small hammer.