The 2007 Mercury Mountaineer Premier's rear wiper is typically controlled by a stalk on the left side of the steering column. There should be a switch or lever on that stalk. Experiment with moving it up and down, or possibly rotating it. One position should activate the rear wiper intermittently, and another will likely activate it continuously. If it has multiple speeds, you might find those settings as well.
If you can't find it on the stalk, consult your owner's manual. It will have a diagram showing the controls and their functions.
