Alright guys, its probably the water valve that leads into the heater core. They have a tendency to seize up during the summer and not want to move when you need them too (during winter). When you turn the heater know to full hot, it drives a motor that opens that valve to allow hot coolant into the heater core.
Its on the fire wall and the lower heater hose leads to it. There is a cable leading to it and the pivot that the cable attaches to also has a tendency to twist, thus not allowing the valve to open.
The thermostat should have nothing to do with the temperature of the air coming into the cabin unless its stuck open. That is fairly easy to confirm simply by driving the car. If it warms up at stop lights and cools off when you start moving, then the thermostat is stuck open. But when you're at a stop light and the car is able to warm up, you should feel the heat coming into the cabin, and then it will stop as the car starts moving.