I recently installed a pair of 6.5" Focal's on the stock plastic speaker mounts in the rears of my R33 coupe.
At first the magnet frame fouled the body of the car. The solution for this was to just swivel the speaker around and redrilling the holes.
The other issue was getting the child restraint bracket in place, had to file the frame slighty for the bracket to slip into place again. The other challenging thing was the position of the bolts and having access to get to them, especially the ones right near the glass. Patience and perseverance is the key!
and presto! see pics.