yes, it can be mounted on a vertical window. mount it as high as you can though, maybe even on the headliner so that the lens is right at the top edge of the glass, so you can get maximum rearward visibility over the tailgate. i had a rear camera (not an a119) installed on my factory-lowered Toyota Tacoma X-Runner and trying to read license plates in the back was a waste of time because the tailgate was usually in the way - see the pic below, taken while sitting at a red light. true, this camera was only 720p, but having 1440p wouldn't have made any real difference except in getting plates on vehicles to either side of me, like the white chevy or older dark colored f150. i can imagine it'll be much worse in your F150 since it's a lot taller than my little tacoma was. The white F150 behind me was not too close - he stopped at a normal, comfortable distance and could have driven around me without backing up if he wanted to.
if you have the sliding center section in the back window like the one in the picture below, you may want to figure out a way to attach any camera you put back there to the headliner, whether it's via magnets or a flat bracket attached to the metal roof inside the headliner that sticks out between headliner and glass (this second method is how i attached mine in the tacoma). otherwise you won't be able to open the window anymore.
one other thing to think about - note the raindrop in the upper left. there's no wipers on the back window, so sometimes the camera is rendered completely useless by rain/spray. Rain-X or just plain old car wax helps, but doesn't completely cure it. same problem for when the window fogs up in cold weather - no defroster. but at least in a truck you have a chance of pointing your center vents at the back window to try to clear it.
@abarth has repeatedly stated that motion detection is working perfectly for him. i haven't tried it yet, but when i get time to dig into the wiring, i plan on hooking it up (with a hardwired battery drain prevention device - a MultiSafer) to be powered full time with motion detection enabled. That said, I'll probably also install a second front cam NOT running in motion detect mode, just in case.