1993 Toyota Corolla Overheating with new radiator and water

My 93 Corolla had a new radiator and water pump installed 6 months ago. Last month my car began to overheat and if I turned the heater on full then the temperature gauge would come down. I discovered that the radiator cooling fan was not working so I unpluged the cable from the radiator fan switch sensor so the fan would run all the time. The car is still overheating until I turn the hearter on. Is my new water pump going south? or is there something goofy with the dashboard gauge? There does appear to be some kind of sensor where the heater hoses come out from the engine block. That sensor has two wires associated with it but I don't know what it's called or what it's for.

Thanks