Lift the rear bottom seat up all the way, feel around for a small metal pin about three inches below the rear shoulder, pull up. It's only like a 1/4" so it might take awhile....
Dude, you rock! That was one of my few disappointments with this truck, when I tried to access behind the rear seat but couldn't.
I need a place to store my "trunk gun" (AR-15). I'll check it out tomorrow to see if it'll fit.