It could be the temperature blend door actuator.
On The AC Case Inside The Dash
rear blend door motor is bad rear blend door actuator motor is bad
expantion valve. this vehical has two
Bad "blend door" actuator or stuck blend door on the driver's side.
First check the fuses then check your blower fan then check the blend door and blend door actuator and then the control panel
the blend door actuator has a gear missing teeth an it can't finish the comand
How do you get to the ac actuator door on a 98 Lincoln town car
Temp blend door actuator malfunction.
The problem is probably in the mixing door not operating properly. It could be in the control panel or the door itself. It just has to be checked out. GoodluckJoe----------------------------------------------------------------------; Blend Door Actuator
Without testing a guess would be a blend door or actuator or maybe a control problem
I have the opposite problem with my 99 Windstar - front vents will not go to heat, which is real fun in winter. I replace the blend door actuator to no avail, and was told the actual blend door was broken, which would require a replacement of the "heater box", for the low low price of $1200. I am currently looking for plan "B".