My Front And Rear Defroster Aren't Working. Does Anyone Know What Could Be The Cause Of This?

Hey guys. I'm new to this forum but I posted a thread a few days ago and I see that most of you all are pretty mechanically intelligent. I have a 2001 Mustang GT and for some reason my heater will only work through the vents. It blows hot air. But the front & rear defroster don't blow any air at all. I'm just curious to see if anyone knows how to fix this. I've worked for NAPA and Autozone since I graduated high school so I'm more mechanically inclined then most females. But this issue has me stumped! lol.
 
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When the heater mode control is changed from AC to heat to defrost do you hear the "hiss" of vacuum? This may be best done with the motor and radio off.

IF not, suspect there's a vacuum leak or disconnected vacuum line between the throttle body and firewall.

Does the temperature of the air change as the temperature selector is moved? IF not, suspect that the heater damper is disconnected or damaged.

The rear defroster is electric and shouldn't have any air blowing on it.
 
When the heater mode control is changed from AC to heat to defrost do you hear the "hiss" of vacuum? This may be best done with the motor and radio off.

IF not, suspect there's a vacuum leak or disconnected vacuum line between the throttle body and firewall.

Does the temperature of the air change as the temperature selector is moved? IF not, suspect that the heater damper is disconnected or damaged.

The rear defroster is electric and shouldn't have any air blowing on it.

Yeah I know that the back is electric, I meant to say like its not working either. But okay I'll try that, thanks for the input.
 
Need a Volt-Ohm Meter. First test. Measure the resistance of the defroster tape. Be sure that the car is off and that the meter is zeroed with a fresh battery. Post the reading.

Next measure for 12 volts at the defroster strip with the defroster on. Post voltage reading. What to do next depends upon these test results.