I have 1999 f-150 V-8 4.6 Last week I was driving and had the defrost on. All of a sudden the windshield fogged up. The temperature control **** has been acting up for a long time. The temp of the truck does not go up. I have the temp. control set to cool, all of a sudden hot air will come out and fog up windows. Then I can hear the blower seem to get stronger and cool air starts to come out again. I replaced the thermostat and nothing happened. I thought that it was the heater core but it is not leaking on passanger side. There is a small leak sometimes from what seems to be the back of heater core. I have gotten chucks of debris from the system. I have seem on here that people heard a clicking noise from near the steering column, I believe I hear something but cannot find the forum with this answer. Please help, I do not want to replace the heater core if at all possible!