why doesn't my defroster and heater not blow hot air

First, you'll want to check to see if your engine is getting to the proper operating temperature using a diagnostic tool that can check engine coolant temperature. If it is not warm enough at the engine, you'll never generate enough heat at the heater core. Next, check the temperature control to see if you can get a difference between any level of hot or cold by changing the knob. You could have a blend door issue there. You could have a heater control valve (if equipped) issue. Hands on testing required.