1999 Ford Expedition Four Wheel Drive Automatic 130000 miles

Hi. I have a 1999 Ford Expodition and the A/C works great, but when you switch it over to the heater it still blows cold air. I was woundering if you could help me. I;m guessing its the theromstat or the heater door not opening. not sure.

I HAVE REPLACED THE THERMOSTAT AND THAT DID NOT FIX THE PROBLEM. I WAS THINKING IT COULD BE THE FLOP DOOR COULD BE STUCK. I HAVE THE SAME PROBLEM AND IT COST 900 TO GET FIX BY THE DEALER BECAUSE THEY SAY THAT THEY GOTS TO TAKE THE DASH APART

yes it is more than likely a mixing door . and they will have to take the dash apart. but I would definitely get several opinions. the dealer is very very expensive and you could save several hundred dollars, going elsewhere. so check it out.