actually, I did the same thing with my truck. If you look up at the driver's side, and then open up the passenger door and look through the access hole, you should see preformed 3 holes, then you can just drill these out as they exactly match a stock mirror. When I looked at installing the mirror on my truck that's what I found. My door was made with the holes in the door frame, but not on the skin - so you should see the holes for the mirror in the frame. It were easy on my truck.
