Installing Mirrors - HOW?

[Ev and the boys]

I hate the mirrors on my truck, and bought some to throw on instead...pretty much the standard fare. Taking the old ones off was a no brainer, but naturally the new ones are a different hole pattern and probably will mount in a slightly different position anyways, but I'm stumped.

I have a backing plate with threaded holes so you can mount with machine screws, but have no idea how to access the backside (inside the door) to install. I pulled the plastic door panel, but there isn't really an opening that will get a hand in there.

Is there some kind of trick to this?