1999 Mercury Sable Heater control not working

We have no A/C any more. Only heat, hot. We can adjust the fan and select direction. Ie: defrost, vent etc, but only hot air comes out. When on "auto" the fan does fluctuate like it should, but after a while we need to manually lower the fan speed. It has been suggested that a flap or motor has broken and the entire dash, including steering colinb, needs to be remover to get at the problem mechanism. Any other ideas are welcome. Thanks in advance. I am hoping we can get at the problem area by cutting athrough the floor or fire-wall. My mechanic is somewhat less than enthused about this suggestion. Lol