To properly diagnose A/C problems it is necessary to answer Mr. Hidalgos question PLUS we need to know high and low side pressure and vent temperature.
On the 300E the heater valve typically fails in a normally off mode. If it is flowing when it shouldn't the problem is most likely the pushbutton control unit, but it should be checked before replacing any parts. You can simply feel of the hose before and after the heater valve. If both are hot the heater valve is flowing hot coolant to the heater core, thus offsetting the refrigerated air from the evaporator.
Get pressure readings, vent temp and give a complete description of what is happening with the blower and maybe we can help further.
Additionally on this particular car, you need to see that air is being drawn into the small grille near the rear view mirror. There is a small fan motor behind the glove box that draws air through that grill, through a tube and to the temp. sensor. If that is not drawing air, all sorts of crazy things will happen.