Rear Seatback removal

[Kneale Brownson]

I want to run some wire from the battery into the trunk to set up a relayed
trailer wiring system.  I decided to "do it right" and take out the rear
seat backrest.  A half hour project has taken all fricking morning.  So
far, I've followed Bentley's advice to remove headrests, release and remove
the real PITA "keepers" that you get to from the trunk and am at the point
where I'm supposed to push the seatback downward and release it from the
retainers "in the heel".  

The backrest won't budge.  I HAVE ALL FOUR of the retainers out.  

I've tried pounding with a big rubber hammer on the top of the backrest,
but there isn't much swing room.  I thought the jolting might release some
corroded stuff.  

Nothing. 

How do you remove this POS?