The way I would approach it is to mount the actual speaker to the door metal. Not the door panel. The best way to do this if you have room is get some MDF (it is a high desnity particle board) and mount the speaker to that and then
mount that to some metal in the door. That helps eliminate unwanted rattles etc... After you mount the speaker then take your original grill and carefully cut the speaker off of the back. You can then glue the grill to the door
panel. Another good thing about this is if you use really nice speakers no one knows they are there.
hope this helps,
Craig
the Gorge VW club