Vaccum line to Heater / AC contols
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On the drivers side directly left of the master cylinder is a vacuum port with a green cap. Now the line directly behind it also reduces to a very small kind of hard black plastic line and goes into the cab using the same heater core port through the fire wall. Here is a picture of the line under that "T" and the little line reduction.
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I don't have a Durango anymore so I can't just go take a picture but if you follow the red water line off the water pump that you see on the right of this picture (yours will be black), where it enters the cab (the one on the right) is where that line runs into the cab then up to the switch.