A headliner can sag and separate on a Volkswagen Jetta because the adhesives holding the headliner up fail over time, which is exacerbated by driving with the sunroof open or in high humidity. Replacement of the headliner can be difficult since the headliner is held up by clips.
You can attempt to use a spray adhesive to reapply the headliner. The cloth mates to a foam pad, which makes adhesive difficult due to the porous nature of the foam padding. You can find various types of automotive adhesives in automobile supply stores or hardware stores.
As this is a frequent problem, a good option is to contact Volkswagen of America to discuss the matter with them. If your car has recently come out of warranty, Volkswagen may cover half or even the full cost of a headliner replacement as a goodwill gesture. You can also discuss this matter with your dealership to see if they will provide partial goodwill coverage.
If Volkswagen will not cover your repair, you should consider using an independent automotive interior shop to do a high-quality headliner replacement that will eliminate the foam padding, which is the cause of adhesive separation. As a bonus, you can opt to use velor, alcantara or other materials and colors to customize the headliner.
