Your engine is overheating.... most likely due to a bad water pump (no longer moving the coolant/water).

When the coolant stops moving, the heater inside the car is always cold.

Check for water pump circulation like this:

http://www.youtube.com/watch?v=thcwatH_BpI&feature=plcp

Wat a bad pump looks like:

http://www.youtube.com/watch?v=WJlS7MvfRXo&feature=plcp

Thomas

Forgot to include a GOOD water pump flow check:

http://www.youtube.com/watch?v=SJN3biTfdGE&feature=plcp

Thomas