I have a 1984 300D Turbo with CA emissions. I am having problems with my A/C and have found that my compressor will not come on because I am low on R-12. There is a pressure switch that measures the charge/amount of R-12. It is located behind the right side headlight assembly. If it is too low, then the Compressor Clutch is not energized. To test this I went to the switch with a jumper wire. With the car running, I jumpered this switch momentarily and heard the clutch kick in and RPM lower. I will be having my system checked. I have been told that it will cost about $250 to have the system leak checked and flushed, and a new Expansion Valve, Dryer, Oil, O-Rings and R-134 installed.
As far as the center vents, mine don't come on with the A/C not running. I will wait to troubleshoot that once I get the A/C up and running. I might have to go into the Climate Control Box and resolder it, but that will come later in the troubleshooting process.