I removed the factory headliner from my '68. Behind the sail panels was more of a jute like insulation. Under the headliner itself was kind of a pressed paper material. Installation of sound deadener/insulation was decidedly not willy-nilly. Factory insulation was intact so I did not remove but I thought about it. The little I picked at it, I thought removal would end up being a huge mess as insulation seemed to fall apart into dust easily.

I wonder if you found a mouse nest?