If you've tried to remove the headrests using the procedure in the FSM, it may help if you know what you're up against. The retainers are integral to the guides, and can be release easily IF you have the right tool. You'll need a thin, stiff metal strip no more than 0.237" wide and 0.110" thick. A stainless steel band from crimp-type hose clamp worked very well for me. Regardless of what you use, it needs to be pushed into the groove in the headrest guide. Those grooves are aligned toward the left side of the vehicle (driver side) and are about 3/4" below the upholstery tuck under the headrest.
The tool should be pushed down the left side of the guides about 2½-3". The groove in the guide passes through its entire length so there is no possibility of going too far. At about 2½-3" the tool will push the lock tab outward and allow the headrest post to be removed fully.
If you go a little farther you can see the arrangement (with the left guide removed):
