I have a Ford Taurus 1997, My windshild wiper work some of the time. What could be the problem?

You have a multifunction switch (wiper switch).
Which could give you this problem.

You also have a Relay in the engine compartment relay number 24 & 25
Look to see if the relay melted the plastic

You also Have a Generic Electronic Module (GEM) Control wiper function. Very rare this unit fail.

Let me know