AC/Heater Question-'70 C20 w/ Factory AC

[Kirk52]

After reading many posts here I am finally starting to figure out which vacuum line goes where and where each cable control connects to but I am somewhat perplexed about the cable that goes thru the firewall and attaches to a flat 90 degree rod that goes down into the evaporator/ heater core area on the engine side of the firewall. When I slide the heater control cable there does not seem to be enough movement in this rod to be able to control anything. Even when trying to move it by hand it seems partially "frozen". I would love to see the end down in the box but don't know how to get to it. How can I see what this rod connects to and whether or not it is working as it should?

Thank you