Evidently the switch arm is not contacting the lever properly to activate - deactivate the motor. The switch needs to be adjusted. Perhaps you can get to it with the motor in but it may much easier to take the motor out. Time fiddling with it in the way can be both annoying and poor use of your time.
You could get a piece of hose and put a stopper such as a golf tee in the end of it and pull the hose off the manifold and cut off the source of vacuum to the wiper motor. Then when it rains put is back the other way. Simple.
I notice that you are from NY. So, therefore, for a more professional looking job get two golf tees. One for the wiper hose and one for the cheater hose.
You're welcome,
Charlie
