Hey man, I have a 79 300D and I have the same problem. I just dont' worry about it now because I know what it is. The vaccuum system most likely has a leak. My whole vaccuum system has multiple leaks that I have to lock the driver door to seal the vaccuum so the engine will shut off when I turn off the key. I am pretty sure you have a leak somewhere as well. I can only use defrost, but that's enough for me. Replacing the whole vaccuum system would just be costly and not worried about it really. I have gotten estimates at about 100 dollars a door plus more for the other areas. I hope this helps, check your vaccuum system.