If the guides are the same for base & sport seats...I wonder if this is the procedure (from online TIS). It also reads as if you should do this from the back seat since they have you tilt the back of the seat rearward...this will give you more room to pull the headrest out of the seat:
Move backrest back.
Move headrest
(1) as far upwards as possible.
Unlock lock
(2) and remove headrest
(1)
