1976 300D Wont change from A/C to heater

I am about to fix a 1976 300D that does not change over to heat when you turn the A/C off. We bought it from a dealer and know very little of the car's maintenance/repair history. I can tell that the car was well maintained and runs/drives great. The heater slide controls are working, but whatever is supposed to happen when you turn the A/C knob fully counter clockwise is not functioning. I was wondering if anybody knew of a common problem that I should look for when I repairing this.
Also, I have not pulled the console out of one these cars for a long time, but I remember that the panel that holds the power window switches was interfering with the removal, but I could not figure out how to remove that panel. Any tips on removing console? Is it best to remove the top and the bottom? What is involved in removing the shifter handle? Thanks for any thoughts/comments you have.

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