How do you replace the rear wiper arm. How to remove the old one?

What year is your Explorer? I'm not sure if there are differences from year-to-year. The rear wiper arm on my 2000 Mountaineer is held on to the splined gearbox shaft with a nut, 13mm if I recall correctly.

Remove the appearance cover to expose the nut, remove the nut, then pull the arm off the splined shaft. Be aware that there is a good chance the arm will be corroded in place to some degree and may require a puller or similar to get it loose from the shaft.

-Rod