I can only guess at this point.

I had opened the moonroof on the ML only to hear a "snap" during its travel. Had a little trouble closing it, but it did.

Turned out the one of the lateral springs that raised the wind deflector had failed. When it failed, it got in the path of the sunroof, thus making it difficult to close.

The dealer only offered the ENTIRE deflector unit ($72), not just the springs. I bought it anyway and replaced the broken one on my existing unit in about 15 minutes. Good as new.

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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