Lack of vacuum to heater control on '89 190E
 

On my '89 190E the vacuum-operated switch which allows engine coolant to flow into the heater for the cabin has not worked since I got the car. The first summer was miserable. The air conditioning and the heater were in a constant battle. In the Fall I got around to testing the switch itself and it was bad, so I replaced it. But I still had the problem. So I then checked the vacuum line going to the switch. There was no vacuum pressure at all (with the car running). So, this last summer, I disabled the switch mechanically so I could get through the summer without additional heat. I was just reminded of this tonight, when I needed heat in the car, and there wasn’t any to be had.

Where does that vacuum line go (at the other end)? I want to trace it back to see if the line is bad, pinched, etc., or if there is some other problem that may be easily fixed.

Thanks.
-Brett