My windshield reservoir sensor began flashing on my dash a day or so ago. I thought the reservoir was almost empty, so I bought a gallon and started to fill it, only to discover it was almost completely full. Up hill or down, the reservoir light on the dash blinks on or off, sometimes staying on for a minute or more, then blinking off. Where is this sensor at, on the pump or in the reservoir itself, and what do I do to correct the problem? Many thanks in advance. Dick