i recently removed the roof rack on my 1986 Olds Custom cruiser to swap it out for a nicer one. Upon re installation, I was having trouble fastening it down. The factory uses these weird rubber things with metal thread inside that gets glued into the roof. Almost all of these came unglued when I started screwing the rack back on, and would just spin in the hole. 2 of them fell into the roof and are now trapped in the headliner. The rack is not securely fastened anymore, and water leaks in through the roof. This has been stressing me out a lot.
Well, at least it's easier replacing headliner in a Wagon than it is in a Sedan where you'd need to go out a door or remove a window. As for a glue, I haven't worked with this kind of repair before. How long do you want this repair to last?
Been there done that. Leave what has fallen in alone, it will find a spot and get stuck. NO need to remove the headliner. Get a threaded insert pop tool, same/easy as a pop rivet gun. before you put the insert in apply a bit of good caulking, I personal;y don't recommend silicone but thats your choice. I'd also recommend doing everyone on the roof, you are there already, rite?