Hi again! I brought my Nitro to another dealer and they told me I need to replace the heater housing unit. They checked the heater core and said it was fine, so they have no idea what the other dealership was thinking. Even though my warranty has expired, they are going to only charge me 15% of the full cost of the repair ($200.00 versus $1300), so I think that's a pretty good deal. Hopefully, this will fix the problem. Thanks for helping!