Inspect the Heater Core

• On most vehicles, the heater core is usually located just behind the firewall, and above where the front passenger would put her legs. Generally speaking, the newer the vehicle, the harder this component is to get to. It looks like a miniature radiator. It has an inlet hose and another hose going out, and cooling fins along the body. Most heater cores are black in color. Inspect it for drips with the heating system on, and if you're able to remove it temporarily, check for major blockage in the pipes by squirting a water hose in one end and observing how it comes out the other. If the water is orange and debris comes out, it might be a good idea to replace the unit.
  
  

Check Radiator Coolant Level

• Without water present, your heating system will not work at all. As the hot water passes through the heater hoses in the engine bay and through the heater core, the heating system uses the heat created to blow the warm air through the passenger compartment. If your cooling system is low on fluid, it will almost surely affect how the heater acts. Make sure the radiator and the overflow bottle are filled to the manufacturer's recommendations before assuming it's a more costly repair.
  
  

Inspect the Blower Motor

• Behind the dashboard or located on the firewall is the heating and cooling system's blower motor. Look in your owner's manual to find the exact location on your vehicle model. This fan creates the force necessary to move the air through the vehicle's ducting, and ultimately through the passenger compartment. If this blower motor goes bad, you may still feel heat seeping out of the vents--but getting it to actually blow out may be impossible. Check the wiring going to the unit, and if all seems in order then remove and check the unit itself. If the unit is bad, it will likely need replacement.
  
  

Inspect Fuses

• Another symptom could be the fuse for the blower motor as well. Consult your owner's manual to see where the fuse is located, and what type it is. On older vehicles, the fuse might be a round glass unit with metal ends, or it could be a color-coded two-pronged fuse. On newer vehicles it's almost always the latter, but check to be sure. Never assume you can fix the problem by simply putting a bigger value fuse in place of the old one. If the factory set fuse is blowing, there could be a problem with the wiring, and if you tamper with things, you risk a car fire. Always replace it with the factory-rated fuse and inspect to see if that solved the problem.