My experience supports Mark's analysis. Most likely to be an issue with the rubber plugs that hold the bow to either side of the roof. That's all that holds the center of the headliner- and getting to them is a DIY affair - so long as your headliner hasn't seriously deteriorated have at it.

Rick

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80 300SD (129k mi) 82 240D stick (193k mi)77 240D auto - stick to be (153k mi) 85 380SL (145k mi) 89 BMW 535i 82 Diesel Rabbit Pickup (374k mi) 91 Jetta IDI Diesel (155k mi) 81 VW Rabbit Convertible Diesel 70 Triumph Spitfire Mk III (63kmi)66 Triumph TR4a IRS (90k mi)67 Ford F-100 (??)