For all who have had issues wth your heating control cables I have finally dug into my car and have resolved the classic issue relating to the driver's side heating flapper door. All of these cars most likely suffer from you driver's heater control slide stuck and no control of the left side heating flapper door.
Since going there and making this repair I wonder how the gents at the factory put these door flappers in the cars when they were new. The heater valve on the heater core seized up and caused a chain reaction of failures causing 2 fractures in the door. Check your heater core valve first through the plug in the firewall. It's most likely seized. The door is probably broken where the linkage from the valve is connected. Don't ask how to remove the door to make the repair. Mine was already broken at 1 hinge. After repairing the door hinge and mount for the linakge (using several days applications of JB Weld I was able to repair the door. The impossible part is reinstalling the door. Does someone out there know how to reisntall without removing 1 of the brass pivots?
I also had no control over the up / down control on the upper right air slide. I always thought it was the slide that failed. "It was Barzini all along....." It was the cable that failed it broke just in front of the control mechinism, probably from the cable being twisted when it was initially put together at the factory. I was able to pull out the broken piece of cable through the mounting hole which has the holding nut on it. I then reused the original cable into the securing hole on the slide (using 15 fingers and multiple flashlights and mirrors) and used some majic to make the original cable work perfrectly. Don't ask me how I pulled this off but I did.
I did have an advantage in that my car was originally and still is "radio delete". This was a total advantage to not have a dash speaker nor radio deck.
In short its a totally diabolical system to work on and make repairs. I cannot fatham how they could effectively put these together when they were new. It's a total phenominom.
I am looking forward to having controls that finally work
I am surpised this is not a hot topic because I know most all of these cars have issues with the above.