There is a blend door that may not be opening the entire way. Thus, only some of the air passes over the heater core. Check to make sure it is working properly. It's located near the heater core. Also, you may need to adjust the cable going to it. Here is a picture of how it attaches. LEt me know if it helps or if you need directions on how to adjust it.
Let me know if this helps or if you have other questions.
Monday, January 4th, 2010 AT 2:24 PM