I have the same problem.
Aparently it is due to a resistor burning out on the heater box. This is located behind the glovebox, which can be pulled out once opened. The heater box is a large black plastic box on the right hand side (the left side is taken up by the fusebox).
Exactly where the resistor is, I have yet to discover, but I am informed by my local garage that it is not an uncommon failing in mondeos. Eventually all settings stop working, which means that the wiring may be burnt out also, so it is worth doing something about it early as poss.
I have been told that the replacement parts can be up around thirty pounds.
Richard Frew
January 2007