I have a 1975 chassis and one of the things I'm adding is a passenger side mirror. I found a 914 OEM mirror and I know there is a " fitting/plate " in the door from the factory for the mirror. I have the interior panel off and I can feel the plate but I don't see how to drill the holes to match the plate? I have read past posts on tapping a punch from the inside to dimple the skin of the door as a guide but I don't see any way to get a hammer and punch in there? Help!!!

use a small punch and carefully punch thru the nut on the inside of the door a "reverse dimple" to see where on the outside door surface where you should drill a hole for the mirror screws.

I drilled pilot holes from the inside out with a Dremel tool. Then drilled the holes with a bit bigger than the threads on the plate from the outside.

Steve

I actually used an ice pick and with its long blade, worked out quite well. A few taps of the hammer and I had the necessary dimples and drilled out the holes from the outside.

There's an idea! I guess i know my small project for the week.

I use an automatic center punch, you don't need to swing a hammer that way. If you drill holes in metal you'll be happy you have an automatic punch after you buy it anyway. They're also most excellent for breaking tempered glass if you ever want to do that.