You've had a replacement headrest, and it's not fixed, hence my suggestion about the interior mirror, as that is a known rattle point, can't see 2 headrests in a row being rattlers, anyway as we can't help maybe just refer it to your dealer.
Ha I feel a Cigar moment coming on.
Right back to the problem, is it only the drivers seat? can you remove the headrest and does the rattle still show? its got to be a matter of elimination and I hope KIA have not given up on solving it.
More than not it will be a small part costing a few pence.
Keep taking it back to the dealer and give them a chance to trace and cure the problem.
ceed has active headrests that are designed to move so it will feel loose but have to say never had one rattle, seat belt top mounts, interior mirrors, sunroof blinds but not headrests worth checking your position in the seat if your seat is too reclinded it causes the active headrest to move sit in the seat and push the brake pedal hard if the headrest moves forward you are causeing the headrest to move off its at rest position and to become active in this position the headrest willmove a lot and possibly rattle to stop this move your backrest into a slightly moreup right position and try again hope this helps
Car been back to Kia and it appears the headrest was not replaced. They replaced the seat back despite me telling them it was the headrest. Now getting a headrest ordered (probably another 9 week wait).