Thank you for this post! This worked perfect on the 2020 Pacifica we just bought!I was looking at forum posts about this issue, hoping someone had figured this out.
I just spent about a half an hour on our new 2019 Pacifica, looking for a release mechanism for the 2nd row stow-and-go headrests - from the back of the seat - with the back cover removed.
I did find the release pin, but it is not accessed from the seat back.
So, after removing the cable release for the headrest, I moved the seat back to its upright position.
If you feel around the top of the seat along the front, just under the plastic headrest rail guides - there is a bump. If you move the fabric down past that bump, you will see a metal clip.
I took these pictures after I had removed the metal retaining clips. The clip fits in that slot visible in the attached pictures, and locks the headrest rail in place. I used a hook type tool to pull the pin out of each plastic guide - and the headrest rail is released. Now you can remove the headrest and the attached cable portion.
There is only a single catch on each of headrest rails, which is the reason they are not adjustable.
Now my Diono RXT's will fit correctly in the 2nd row seats, with the headrests removed.