Mercedes Benz repair: air conditioning, air exchanger, 300sd

Question
my 300sd 1984 fan works fine but when i go to press the button for face fan it stays stuck in the defrost blower stage.  i try to get it to switch to the floor and it still stay there.  there is no cold air and it has been sitting for about 5 years.  fairly new engine only 75000 mile (replace it because of timing belt broke) and the body is in good shape.  but the air is a problem.  i have had new freon put in a few year ago and it put out fairly cold air but the air guy said it might need a new air exchanger who knows!  i love the car but am now restoring it slowly.  runs great.  got any ideas?  also there is a clicking sound next to the air control buttons that just vibrates periodically.  any ideas?

Answer
Sounds like several possible issues, each would be massive typing for me.
The simple answer is for you to join
http://www.mercedesshop.com/shopforum/forumdisplay.php3?f=15
Post an introduction thread in "Diesel Discussion" with your current issue, ask what search words to use.
There are hundreds of pages + pictures and DIY on the A/C system.