Another thing to consider is that the thermostat may have failed in the open position. If the thermostat is permanently "open" the engine will be VERY slow to warm up and may not even get off the "cold" mark, especially in cold weather.

-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