The factory ones are pop-rivet mounted.
The rear ones have a somewhat (1/4" diameter?) smaller mounting triangle than the fronts. Plus, it's a smaller door hole in back than front.
When I put front speakers in the rear, I have to drill new holes for the pop-rivets (actually, I simply enlarged the holes inward into an oblong oval) and remove the water guard to get it into the holes.
That was originally intended to be a stop-gap till I got new speakers (after being rather disappointed by some 6.25" Pioneer ones) and the Bazooka bass-box.
Now, I am running the Bazooka and the 4 front-door speakers and I am rather happy with the results for everyday use. It isn't perfect, but fine for what I am using and sound decent.
The holes are rather large for 6.25" speakers and one NEEDS an adapter of some sort to make the speaker mounts not be inside the door speaker hole.
For 6.5", the rear fits OK with a self-tapping screw, but I can't remember if the front will or not.
Regardless, when I DO finally get new speakers, I want to pop-rivet in an adapter plate (wood or plastic), then screw-mount the actual speaker to that.