Just to follow up on this and for the benefit of anyone else in future, I successfully installed the new speaker grille this afternoon, thanks largely due to Flapdog's guide on how to remove a front door card on the 159
I didn't need to take the whole door card off (involving disconnecting wiring), taking out all the bolts and loosening it off was enough to do the job.
Here's Flapdog's guide copied again, but tweaked slightly for the differences I found on my Brera and for only loosening the door card instead of taking it off completely, differences highlighted in bold.
1. Theres a plastic tab behind the door handle that needs to be removed, these should be replaced as you're likely to break it (around £1.50 from AR). It seems the previous owner of my Brera found this out - the tab was held in place perfectly well by a blob of blu-tak
Undo allen bolt behind.
2. Remove 2 allen bolts from arm in door, in front of window switches
3. Undo 4
screws on very bottom of door panel and smaller screw on the side of the door card, above the lock
4. Remove triangle trim at front of door, not part of door panel, can be removed with flat blade screwdriver, but needs care to do this.
5. I also removed the 2 rubber grommets on the underside of the door just with my thumb and finger, either side of the puddle light. I then removed the nuts underneath though once I'd pulled the door card away, I don't think they were holding it on.
6. Gently but firmly pull door panel away from door, you don't need to pull away the top which is secured into the window channel with 2 tabs and difficult to remove
. The rest of the door is held on with push fit pins.
7. With the door card loose there's enough room to pull the door card away from the door and get your hand behind the speaker grille. From there you offer up the new grille, push the metal interference fit features in the door's foam lining onto the two plastic pins at the top of the grille. These little metal features are like washers that the speaker grill pins push through the middle of, the metal cuts into the plastic to hold the grille in place
Put everything back together and it's job done.