I grabbed a few HVAC pods at PickNPull from a W123 recently since the dash was off and they were visible and easy. I recall one at the top acts longitudinally on the car's centerline to open a long door w/ horizontal axis running cross-wise (left-right). That door probably goes to the cowl vent. The pod was easy to take off and I recall it looked like one might be able to do so with the dash installed. You would probably have to remove the radio and push-button switch to get to it.

Behind the push-button are ~5 vacuum-electric solenoids that control all the pods. You can find pages from a 1983 manual here if you search that explains how each solenoid actuates in each climate mode. Allow for the translation to English. I recall the solenoids are called "code" 1 thru 5 or some strange term.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans