I have been fighting this problem since last fall. I found that the cable that acuates the door between hot and cool failed. something in the linkage broke or the lever broke the result was hot air only. removing the dash was the only way to go to find out what was the cause, big job, so I found a temporary solution and I thought I would pass it along. At the bottom of the swing door is a shaft that the door swings on, I put a lever on the shaft and move the lever manually and it works fine. when I get the time or courage or the heater core fails I will remove the dash and fix it permantely.