You should remove the interior door panel (or door card as some call it). Besides the clips around the circumference (some also have a screw which should be obvious), the plastic "bezel" around the door handle holds them on. You can carefully separate the upper and lower halves of the door handle bezel to take them off. Then you can see what you've got. I'm sorry I don't have a clear memory of what's behind the door card. Since I suspect most, if not all door mirrors are designed to be attached to the door from the outside, you should also make sure you can mount them the way you are planning. It might be possible to use a machine screw with a nut and washer inside the door but I doubt that will be the intended method.
I don't remember any access holes in that area of the door. If you're really motivated you could make one. Conversely, you might be able to reach up from the speaker hole. I THINK that most mirrors come with plastic pieces that you insert in a hole in the door and screwing the mounting screw into them, they spread out locking them in place (think molly bolt).
I hope that helps! Good luck!