Hi there! Same issue.

93 300E. Headliner has been sagging for years...more pertinent issues to fix, so I've lived with it.

Tried the 3M spray adhesive...worked for about a minute. The deteriorated foam won't allow proper adhesion. Only a replacement will do.

Texas summers probably accelerated the breakdown of the foam...

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car