Rear headrests have to be in the down position also.

When you release the top lock, the canopy has to be raised just past your head before the automatic operation will engage...

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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