Hi there,
I second utnj's suggestion to go to his site and download his vacuum diagrams, they were very helpful in locating vacuum leaks in my '79 SL's system. One thing he doesn't mention but I found wrong was the one way valve (Blue body, black end) that is under the hood on the driver's side of the car next to the hinge. On my car it was hardly passing any air, so the vacuum level in the entire climate control system was low. Also, I ordered some 3 way and 4 way rubber vacuum connectors from Performance products, and replaced all the ones I could find, as they were getting hard and leaky. If you don't have enough vacuum to your main switch (Sw 19) the fan won't work in Auto lo, Auto Hi or Bi-level positions. It takes around 175 mbar of vacuum to close this switch, and it is in series with the fan motor. In the Defrost position the switch is bypassed by a relay contact on relay 18. That connector on the bottom of the control unit with the two vacuum lines you mentioned in your post is a thermostatic switch that only allows the compressor clutch to operate after the engine temperature has reached at least 40 degrees centigrade, or around 105 degrees F, so if you don't care if your compressor operates at a lower temperature, you could just bypass the thing entirely.

By the way, I purchased and installed one of the Unwired tools kits, and it works just fine after I fixed my vacuum leaks and fixed a crimping error on the +12v line at one of their connectors. They offered to replace the unit, but I had already fixed it. I recommend their kit as a very good fix for a poorly designed original control unit.

Regards,
Richard Wooldridge
'82 300D/4.3L V6/T700R4 engine/tranny swap
'79 450SL
'75 280C