you will really struggle to reglue a saggy lining.. i've tried all sorts..
you can use pins to create a quilt effect
Its the Sponge that has disintegrated.
the only real recipe is another liner board from another car which will be likely to fail as well due to the age.
or take out your liner and clean off all the old spronge and recover it with new material , loads about to chose from , but i've not found any that are a close match to OEM .
C