hi-strength 90 - it comes in a green can. I used it for my plad headliner 4 years ago and it still looks perfect with no sags, its more than the adhesive though, technique is important, im going to do a write up on this soon.
dont do this on a tight budget get at least two cans the job is a pain and its not fun to do it twice.
good luck, pm with questions i have done many-a-liner
Adhesive seen here:
this has motivated me to do a write up for the forum, stay posted, im gonna try to do it tomorrow as i just bought a 89 gti with saggy liner syndrome