Mine is a Ford, but the need is the same. I wanted to hang a fair sized two way radio on the ceiling. My truck has a cardboard headliner, but a double wall steel roof under that.
I was scared of nut-serts due to the weight, 4 pounds or so, and the pounding of running up and down roads with potholes and offroading. Since the attachment would be covered by the headliner, I would not see any
cracks or impending failure.
I cut a thin slot in the first layer of steel where it would be covered by the radio. Of course, it would also be covered by the headliner, but I could run with the headliner off and not see the cut. Then I marked the holes for the radio mount a few inches away from the slot. I drilled the holes a little oversize.
I cut a piece of 1/4 inch steel, about an inch wide, and about as long as the radio is wide.
I marked, drilled, and tapped holes in the same place as the ones in the mount. Plus a couple of smaller ones in between.
I ran a piece of wire in one of the holes in the truck, and worked it over and out of the slot. I lightly hooked it to the steel piece, and slid it in the slot. Used the wire to pull it over. I worked it around until one of the other hole lined up, and put in a screw.
Then, using a stiff piece of wire, and letting that one screw hold it lightly, I worked the piece around until the two little holes in the center lined up. I put in two countersunk screws to hold the backing plate in place.
It was easier to do all this than it sounds. I also tied a piece of string to the plate from the outside before I put it in the slot, in case something went wrong, I could pull it back. After it was mounted, I just broke the string.
After that, I mounted the radio mount to the roof using the two larger screws screwed into the backing plate. Cut two slots in the liner for the mount arms to come through, and mounted the radio.
In over 10 years of hard pounding, it has never even worked loose. Since the 1/4 inch backing plate is about 1 inch wide and about 8 inches long, there is a lot of surface area to distribute the load. If I ever want to change
the mount, I can take the two mounting screws out, and the two countersunk screws will hold the mounting plate in place.