Rain sensing wiper problems.?

The sensor is in the front of your rear view mirror facing the windshield. Make sure it is clean and clear of anything that could block it. Depending on the age and model of your car it could be in various places but is usually found closer to the penger side of the vehicle. A dry Q-tip will usually get any dust off the lens. If the problem persists try the wipers with the blades in the lock out position. Use a garden hose to wet the windshield. Clear the windshield of moisture manually and if the wipers keep going you know you have a system failure rather than poor blades. I have given my rear view a sharp tap to get the system back online and it worked for months. Good luck.