It might be possible that the headrests have been taken out before (like described above) and haven't been pushed back. The pins of the headrests have to be clamped into the plastic gearing and as long as they go up perfectly, I doubt that the gearing is worn. Figure out whether the headrests are clamped in the plastic gearing (they go up because they rest on the plastic gearing but won't come down because the plastic gearing doesn't hold the headrest pins.

greetingz,

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1990 300SL-24
1993 C250D with a minor 600+k kilometer
www.MBenzNL.com(the Netherlands)