Merc Cougar has very little heat

I have a 99 cougar that I'm having a problem with the amount of heat that will come out now that it is cold outside. On the lowest fan setting the heat comes out hot, as it should. On the second setting it blows a little less hot and on the third setting it is warm at best. On the highest fan speed setting the air is cold. This happens on all settings, vent, feet, defroster, it's all the same. Is it possible that the heater core could be partially blocked and only flowing a small amount of coolant through it? Could the thermostat be causing this? The coolant temp gauge is not running hotter or colder than it usually does, the level of coolant in the overflow tank is good. There is no radiator cap on this model car, the overflow tank has a pressure cap, so I can't check the coolant level in the radiator itself. The first fan setting doesn't cut it now that it is 15 degrees outside.