Hi there,
It is a vacuum line, just take a piece of plastic line and plug the two rubber hoses into it. This will allow vacuum to reach switches 20 and 19 whenever the climate control is set to anything other than the off position.
I had to do quite a bit of rework to revive the heating in my 450SL. I bought the car on Ebay from a dealer in Fort Lauderdale, FL, and apparently the heater had not been working for several years or more - the control unit was all corroded, the gears and drive to the valve were completely corroded away! (I found that the heater didn't work when I was driving the car home to Washington State from Florida, when I got to Eastland, Texas it was 23 degrees, and NO heat! The AC worked great, though!) Also, the vacuum plug that plugs to the top of the control unit had hardened up and the lines were very leaky where they plugged onto the control unit. When I installed the digital servo replacement I ended up removing all that, just have 4 vacuum lines coming through the firewall now that connect to some new vacuum solenoid valves that are operated by the digital servo control unit. What I did was find another donor car in a yard and I stripped out all the vacuum lines I could, ended up replacing much of the vacuum system. The digital servo upgrade simplifies the vacuum setup, and I redid the vacuum lines wherever I could because the Y and cross rubber connectors were all hard and leaky where the hard plastic lines plugged into them. I also replaced the pushbutton switch and the group of lines that plug into it with the one from the donor car as it was in much better shape. I think my car was parked in a hot place a lot to make those lines go bad.
I ended up using a mity-vac to check all the vacuum elemnets and switches for leaks, and found many. It only takes one leak to kill the heater operation! All the vacuum to the climate control unit goes through tiny holes in the push button vacuum switch (around .030!) so it doesn't take much of a leak to cause the system to fail. I borrowed a factory manual and used it in conjunction with the Unwired tools downloaded vacuum charts, which I found much easier to read. The ACII climate control is amazingly complicated and hard to service since it uses vacuum to control both vacuum and electrical switches AND a differential amplifier to run a motor that operates both the heater valve AND a vacuum switch AND electrical switches that control the fan speed. It took me a lot of evenings reading the factory manual's ACII section before I really understood the function of most of the pieces of the system. (Still don't understand the function of Relay 13 - something to do with the AC system.)

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