Car running too cool.

I replaced the water pump, thermostat, hoses and flushed system. Holds pressure. Ran cool in summer now really cold in winter Just above "C". Very little heat on hot setting. I blocked off 3/4 of radiator and runs just a little warmer but not much. I did notice the electric fan turns on when thermostat is one click above "C" it then cools down and shuts off. Could it be Coolant Temp Sensor, Temperature Switch or Relay for Cooling Fan Motor? In the summer the heater worked better and temp on temp sensor was running hotter. Thanks!