I searched and didn't get any hits on a seized rear wiper, so posting this.
Rear wiper on my 2014 GT had been slowing for weeks, finally stopped all together.
Took it out of the vehicle. The wiper arm shaft seizes to the body it goes into. There is a little rubber seal, but not that effective. After sanding the arm, and driving it out with a hammer, cleaned up the arm where it goes into the body of the assembly. Lubricated well, and reinstalled. Will see how long it lasts.
Typically rust belt problems.