I have a 2004 Chevy Blazer not a Trailblazer. There is NO HEAT coming from the heater. I have changed the thermostat and checked the coolant levels. The temperature guage on the dash says the engine is running at 200 degrees while driving. The thermostat I installed was 195 degree. The car is not overheating at all. All of the HVAC fuses are good both on the side of the dash and under the hood. The heater fan works fine as do all of the selector switches. I assume the water pump is OK or the engine would be overheating. What else could be causing the no heat problem.
The first thing to check is that both heater core hoses feel the same hot temperature. If one hose is cooler to the touch than the other then you need to remove both heater core hoses and flush the core with a garden hose. Alternate back and forth between the two pipes on the heater core until the water flows out clear.
If they are both the same temperature AND they are HOT then you likley have a problem with the temperature blend door inside the vehicle. It may not be pulling to the proper position.
Both of these are common blazer problems.