I've just picked up a new to me 79, and one of the main problems with the car at the moment is that the vent selector does not seem to function properly. When you move the slider, it does sound like it's trying to change from heat to vent to defrost, but at the moment it seems to be doing heat and defrost at the same time regardless of the position of it. What is the best way to go about troubleshooting this issue?

 

check the mechanical linkage to the plastic flue valve that directs the air through the different HVAC pipes

 

most cars will failsafe to defrost.

 

What is the easiest way to get to that?

 

Reach up behind the dash to where the lever connects, you'll feel a cable that slides with the lever... Follow that cable to where it goes. 9 of 10, it has either become disconnected from the lever or the valve at the other end.