Once upon a time a previous owner installed some big truck mirrors. Holes in the door and top of the door above window have been welded shut and doors refinished. I plan to buy and install the correct mirror. I recall , or think I do, that someone posted the "template" that is used to locate the holes for the mirror on the cowl. I looked inside under the dash and it does not look like the "optional" mirror was ever installed. Was there a "backing plate" for that mirror that went on the inside? Advice, suggestions, anyone ?