To get a better understanding, and more specifics of the symptom, I will ask you some questions.
Do the wipers turn off when the key is switched off. If so, do they park at the next key cycle, or never park?
I assume that they just come on after driving, or immediately when the key is turned on? Are they capable of changing speeds when running, i.E. Low, high, and intermittent modes?
When they come on, what speed do they come on at initially, low, high, or intermittent?
Lastly, does the washer pump run at this time? Does the washer function normally?
There are two possible components capable of causing this, the wiper(multifunction) switch, and the windshield wiper motor, which is a module. Your next step in the diagnostic process may require the switch to be disconnected, this would require some steering column teardown, is this something you are potetially going to want to tackle?