Well, after having to deal with this myself and seeing that tons of people are still having the same problem, I decided to do a quick 5 minute walk through...yes it only takes 5 MINUTES!!
If you notice that after 15-20 minutes that your car STILL isn't warm, even if you have the heat on full blast, it doesn't necessarily mean that your core is gone. The problem that most people have come across (myself included) was the issue of a small black clip popping off of where it should be.
To FIX this problem....all you have to do is push your PASSENGER seat all the way back, stick your head under the glove box and look to your left, behind the kick panel. What you will most likely see is what I have pictured below. A white plastic piece protruding from front to back, and most likely the clip dangling on the HVAC cable. Just maneuver it til the open side is facing the white plastic & clip it back on. Tada...that should be all.

So...in essence: If you turn your temp **** to the right and the clip is off, it doesn't completely switch from Cold to Hot and your cabin will never get warm. When it's clipped back on....you will DEFINITELY feel the difference.
regards,
-Phil

 

Good detective work.

 

You know this man....

 

I believe that's fixed my issue.
My fiance' got his head under there and saw that the HVAC cable was barely clipped.
Now it's clipped real good and the heat is HOTTTT!!! Im so f'n happy!!! *jumps for joy*

Thank ya very kindly sir!

*curtseys*

 

You're welcome!