Switch OFF, wipers ON...

I'm working on an '08 CTS. The wipers and window washer come on intermittently even though the switch is off. Testing I found a square wave signal on the wire from the washer push button switch to the body control module. All of the other parts of the circuit (low, intermittent, high) are pull-down circuits and therefore have the typical system voltage at rest. When a switch is activated, the voltage is pulled to ground.
Has anybody ever seen this square wave?
BTW, all normal functions work (mist, low, intermittent, high)... when it's not acting up.
Thanks,
Vince