I have a 1998 Mark 8. I waited for the engine to warm up, saw that it was by the temp gage. I turned the heater on to the higest setting(90 degrees) nad no heat flows out. The fan blows well on all settings(vent, def etc). I hear the A/C doors moving just fine, but no heat is blowing at all. I found a web site that talks about a way to diagnose the system by pushin a series of buttons.
John