Also, see if the light on the rear headrest switch is blinking...the rear headrests need to be parked in the lowest position for the top-down operation to engage.

This should occur automatically, but if not, it could also be your problem.

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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