If you can't easily see where they mount you probably have the headliner covering the hole. On each side there should be a large hole in the metal structure surrounded by three small holes for the screws. When I mounted mine I was going thru a fiberglass headliner but knew the approximate location of the mounting surface. I used an ice pick to locate and then open the holes. I don't have a photo of before installing the headliner, perhaps someone can supply that. In the mean time perhaps these photos (if they show up) will give you the approximate location of the underlying metal mounting surface.

[img]https://www.stovebolt.com/ubbthread...d/Number/21243/filename/IMG_3008.JPG.JPG[/img]
[img]https://www.stovebolt.com/ubbthread...d/Number/21244/filename/IMG_3016.JPG.JPG[/img]

Chuck

1952 Chevy 3100 1/2-Ton
In the Stovebolt Gallery
More pictures on Photobucket