I will admit, when I first very briefly looked into this, I stupidly believed all of the posts where people said it was not possible to install 2nd row headrests. I wonder how many others have wasted time and money unnecessarily replacing entire seats or interiors because of wrong information posted all these years.
It is a small amount of work to remove the seat cover/foam to cut the necessary holes in the seat cover/foam to allow the installation of the headrest guides. It took me about an hour to do all 3 in a 16 PIU. There is no need to remove the seats, it can be done by simply adjusting the seat forward or backward as needed.
Black headrest guides - need 3 of each
CU5Z-96610A16-BK
CU5Z-96610A16-BR
Headrests - you need to source these yourself - can be new or used - if new, I believe the headrest and the cover are sold separately - you can also buy any used headrest and new covers
The foam appears to be hog clipped to the seat cover, so you need to reach in to cut it, or you may be able to pull the cover back enough to expose the foam enough to cut it. A tool to remove auto push pins is helpful as there are a few securing the seat cover on the back side. Once you remove all the clips, the seat cover and foam can be pulled up and off the frame, just wiggle and move as needed until it comes off.
Anyway, here are a few pics. I didn't detail the entire process, because it went so quickly and it's pretty cold in MI right now.
Seat cover/foam removed from 60 side with guides partially installed to show the seat frame in PIU is provisioned for them - the guides will actually be installed last, through the seat cover. Be careful when testing fit while trimming the hole in seat cover that you don't fully snap them in place until the hole in the fabric is cut properly.
Holes cut in foam to allow guides to pass through.
Seat cover/foam removed from 40 side
Cut holes in foam to allow guides to pass through. Hard to make out in pic (faint circle on right side), but you may have a mark where the holes in the frame are to aid in cutting in the right spot.
Pic of headrests installed in 16 PIU.