Several factors could cause your truck's heater to blow cold air instead of warm air. It can be a Low Coolant Level or Coolant Leak because the heater in your vehicle works by using hot coolant from the engine. A coolant leak can also lead to insufficient coolant levels meaning there might not be enough to circulate to the heater core, resulting in the cold air you feel. However, the thermostat regulates the flow of coolant through the engine. If it happens to be stuck open, the engine may not reach the optimal temperature, causing the heater to blow cold air. If no issue is found with the thermostat it may be the Heater Core. Fixing any of these probable issues begins with a proper diagnosis, you’ll need do inspect the truck for the causes to the issue, you may want to start by looking at the coolant level, you want to make sure the coolant is not leaking first, because if its leaking that is where you want to start. If the Coolant is just low, you can start by adding the correct coolant for the trucks make and model. But This may not fix it, you may want to inspect the trucks thermostat, when the trucks thermostat malfunctions it can prevent the engine from reaching the optimal temperature, which is required for the heater to work effectively. If you’re looking for a mechanic who can do a diagnostic test and evaluate the performance of the Heater Core.