I have a 2003 S-Type with 4.2L V8. 136k miles. It is having a heat/AC issue I've never heard of. The heat works beautifully. The fan controls work fine and you can vary the flow of the air from the vents. The real issue begins when you switch to A/C, on this model there are 4 vents on the dash. (2 in the middle side by side. And 1 on the far left, 1 on the far right.) When I turn the temp control down and the A/C kicks in, only one of the middle dash vents blow cold. The other 3 vents all blow hot. At first I thought this might be the DCCV, but after more consideration it almost seems like the climate control module. Let me know what your best guesses are. Also this is the model of climate control without the navigation.

 

Have you previously replaced a failed DCCV? That's characteristic of either burned traces on the climate control module (normally caused by failed DCCV) or failed temp sensors. If temp sensors, they should be identified by a code-read if you have access to a reader or someone who has one (WDS/IDS, et al) that will read body and chassis modules (beyond std OBDII powertrain stuff)
Do a search on climate control and you'll find a wealth of information on your exact symptoms.