Update & Warning: I was finally able to release seat back. At the bottom right corner of the seat back, there is a rounded plastic/vinyl cover on the inner side of hinge that I could push open with a flat head screwdriver & reach in with another flat head & trip the seat release. I noticed that the U-shaped slot that holds the end of black plastic cable sleeve was not "snug" enough to keep cable in place when cable is moved about. I think the cable fell out of slot the last time I folded seat, or put something under seat, or when I put vacuum hose under seat to clean. When it popped out, it looked like it got caught in between something & was cut/pinched into. Since I really want seat to work far into the future, I put a drop of crazy glue where the slot & cable sleeve meet, being careful not to get any on the cable. Hopefully this will prevent this from happening again. Thanks for the read.