Can anyone give me information on what is on the upper side of the headliner (under side of the roof) where the roof rack hold down nuts are? I would like to drill large access holes, to be filled with electrical panel plugs, so I can tighten down the roof rack without pulling the head-liner. But I would like to be sure of the size and placement of the holes before I start drilling, as I would like to do it only once and not go through wires or the body. I have Forstner bits, which would make a very clean and shallow hole, but enlarging such a hole is practically impossible.

Unrelated to your question, but a hole can fairly easily be enlarged by using a jig with the hole you want clamped (somehow) over the hole you wish to enlarge.

That's a good point. Take some wood, drill the hole size I want, then use that as a guide.

Maybe I could do a small hole in about the right location and peek inside with my borescope.

Should remove the headliner.

You want to avoid the air bags.

Well yeah.

Aren't the air bags by definition not behind the headliner? If it is made of the same stuff as the door panels, it's quarter inch fiberboard. Don't want that impeding, or being propelled by, the exploding air bag.

The easiest way to enlarge a hole, is to use a "step bit". Some call it a "Christmas tree bit"

Of which I own many! AvE turned me on to them a while back, and my life was transformed. The holes I am looking to open up are a bit larger, though. For that I have a Forstner bit set.