Need help with Expedition rear wipers

Recently our re wippers quit working. Any I deas where to look or any histrical problems?

Going to need the year. Each series works differently. My '05 it was the shaft that extends through the window. Whatever excuse for grease they used, totally gummed up into a solid mass. Found that out by having the window open and the motor worked but couldn't get the wiper arm to move by hand.