I need help. I have a 1998 lincoln Navigator with 117k on it. I've had great heat until today. I can have the heat to 90 and it feels like the a/c is on. The only thing I've checked so far is the heat is still working in the rear. It was to dark to check anything else. I did some searching and found the things I hope it isn't blend door and heater core. I was hoping someone could give me some info so I could check in the morning. I found it strange that it went from working great to not at all. There's also no smell of antifreeze in the car. Thanks for the help.

 

Sounds like blend door problem