I have a 1971 4020 that I recently purchased and intend to use to feed round bales in the winter. It has a Hiniker cab that I cannot get heat out of. I have flushed the heater core, lines, and cooling system and added new coolant. I have also installed the high temp thermostats and put cardboard in front of the radiator. The shutoff valve going into the core opens and closes properly. The high temp line is plumbed into the top of the oil cooler and the return line goes back to the lower hose. The tractor was run at 5 degrees F today and blows ice cold air at idle and cool air at throttle up. I can feel warm coolant going into the core and barely warm coming out of the core, so I know it is circulating to some extent. I would only like to get enough heat out of this to keep the windows clear in the cold weather. Any help would be appreciated.