Ford has had problems on many of its models with the "blend door" actuator failing. The blend door is what determines the output temperature. It sounds like your blend door has broken, and some air conditioned air is going out the passenger outlet, while the other outlets are receiving ambient air.

-Mark- '99 Mystique LS, totally original (including the original water pump and the premium stereo)! Bought new in December 1998 for $21,000. 208,000 miles