Re: What are these holes in my Suburban door for?
 

RoyL, I have the same problem on my driver side door. Someone bondo'd the original holes and mounted a car mirror farther back. The good new is that the original holes are still visible through the finish, so I hope that I can use a small drill bit to drill through the bondo and then run a tap through the holes to clean them out. The passenger side has never had a mirror mounted, but does have the backing plate. I think I will try your center punch trick on that side since I have a small spring loaded center punch. Of course then I have to fill the car mirror holes...

Re: What are these holes in my Suburban door for?
 

I got the driver's side done after making a template with tape using a buddy's GMC. Used a tap as you said to clean out the bondo. It turns out that the orig holes had been welded up not bondo'd, so I believe my center punch mark wasn't able to show thru to the outside due to the extra weld material...

I used a 5" center punch, would be better to use a shorter one, you need long skinny arms to reach the 2 closest backing plate holes, unless you have the glass out...

Re: What are these holes in my Suburban door for?
 

Added diagram. This will get you close (I measured 3 different doors and none were the same, so I show the range). The door edge and body line are not straight, so hard to get good measurements … is best to locate holes from the mirror backing plate inside the door (if your truck has one).

http://i1100.photobucket.com/albums/...ps04e4ae12.jpg