windshield wiper problem

[wims66]

I have a strange problem with my windshield wipers.
My '66 chevy truck has a 2 speed selector with some kind of pushbutton in the middle of it.
The problem is that when i am engaging the wipers both of the speeds work,but i can not shut it down.
When i put the butten in off position they will even wipe faster.
Sometimes when the voltage drops the will go slower ,and when the windshield is not wet anymore they will shut down.
What could cause this problem,does any one have a idea?