My 89 GTA hardtop headliner is bowed down about 2-3" right over the rearview mirror. I've got it out and its the plastic backing that's keeping it bowed. Has anyone had any success straightening one of these? I've thought about heating the plastic with a hairdryer and then clamping it between 2 boards, but I'm kind of afraid of damaging the fabric and/or having it come apart

 

Sounds like this is a newer replacement headliner that does not have the front clips like the original. While the instructions don't have anything specified to secure the front, we put 2-3 two inch square pieces of industrial Velcro along the front to hold it up. Put them where the slots are for the original clips. Haven't had any sag in mine in two years. Did the same thing with my son's this summer.

 

I have it out of the car. It has a pretty significant bow in it. I don't think velcro is gonna work. the plastic underneath is fairly stout, so I'll have to do something to straighten that out.

 

 

You might try a heat gun to heat the plastic and bend it back into shape.