The rear seat doesn't fold down in the Sierra. You can remove it by pulling some release locks that aren't the easiest to get to but there is no "fold down" like what I think you are looking to do.
They are lots of youtube videos that I watched to figure it out. Bent ice pick works good. Once it down. Put some zip ties loosely in the hole to grap with your finger, for the next time
Before I go through all that, I'm trying to troubleshoot my 120V inverter. Like an idiot I overloaded the bed outlet and it shut off. The fuse, #19 according to the book, tests good. And, unlike the book says there is no switch for it.