the speakers in the front need a 3 hole to 4 hole adapter ring that is a spacer... I modified my factory one, a lot of people buy them from Crutchfield.

The rear if you use a 6x9 needs a spacer to space it out so the magnet doesn't bend your fender out.

The spacer is just a piece of wood or whatever to move the frame of the speaker out of the door or fender a little.