Oh, sorry. I was replying to your reply to LittleBlueThing's suggestion of mounting mini-speakers on an internal shelf.

For car speakers, the hole in the drywall would be cut smaller than the rim of the speaker. They then just screw to the face of the drywall (using plaster plugs) in (typically) 3/4 places, sitting proud of the wall surface. The grills then usually just snap in place over top of them.