If you get up under the lower dash panel you can see where the valve on the ignition switch is. Unfortunately I don't have a good picture of it. Just one thats pretty darn fuzzy.

As you can see in the other picture, the yellow vacuum lines go to the door lock system. The green w/yellow line goes to control the AC pods.

The first thing I'd try is to plug off the AC and the door locks. (Golf Tee's work great for this, as you can see in this picture my wifes door locks are plugged). See if you can shutoff then. If not, than the problem is elsewhere and could still be a number of things other than the shut-off valve on the IP.

You'll want to get a vacuum pump (MightyVAC). It makes tracking down vacuum problems so much easier and will also allow you to test out the shut-off valve on the IP and the one in the ignition.

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'84 300CD Turbo 132k (Anthracite Grey) - WVO - My daily driver - Recently named coo-coo-coupe by my daughter.
'84 300D Turbo 240k (Anthracite Grey) - Garage Queen
'83 300D Turbo 220k (Orient Red) - WVO - Wifes daily driver

I'm not a certified mechanic, but I did stay at a HolidayInn Express last night.