I fitted headrests to seats 6 & 7 to make for better safety for Grandchildren who reckon that riding in "the boot" is the bees knees.
Got the headrests out of a scrap Rover 400. They are gray velour, my car is gray leather but they look good because luckily the shade of gray matches.
The rests have two long chrome stems each. The frame of the third row bench is rectangular in section. You can feel the shape of it through the covering. The top horizontal is wide enough to drill to accomodate the stems of the rests and still leave enough metal to be sturdy; there are two layers to drill through, these layers are about 15mm apart which, when drilled, gives a tubular effect which provides fore and aft rigidity for the stems.
The cross section is slightly concave, which helps to locate the centre. Sort of valley effect. You can feel it and quite easily guage the centre of the "valley".
Carefully measure and mark the top of the seat to position the stems to suit the seat belt / sitting position centre locations then ( with courage in both hands ) use a centre punch to define the target then drill pilot holes through the fabric/leather then metal frame followed by interferance fit holes for the stems to be pushed right through the top seat frame section. The stems finish up hidden inside the seat back cushion but there is enough padding to prevent the stems from being felt through the seat back rest.
If you take care to drill the interferance fit holes so that they are a good " grippy" fit into which you have to push hard to insert the stems, then the rests stay put even when the seat is folded. A bonus is that the rests tuck neatly into the gap between the third row seat top and the car floor when folded---you don't have to remove to fold---just the job.
Hope all this makes sense--- any further info required email or PM might be easiest.