Maybe tough. Suggest you contact GW and see if they will give you the contact info for a couple of ex-GW employees that started their own business fixing headliners on GWs. They travel the country and do the field work for GW now. In the NE it was found that the expansion of the fiberglass structure and the composite headliner are different and depending how attached, cracks occurred in the headliner. They remove, repair and reinstall headliners and probably other warranty work for GW. They should be able to give you some ideas on how to get access and remove and re-install the headliner if you have to. Worth a shot.
I had a crack situation and they did mine a number of years ago at the dealer (I did not see the repair or meet them). They fixed it and I cannot tell that it was done. I understand they also reattach the headline a different way to mitigate recurrence.
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