There are a couple of bolts along the bottom of the window that hold it to the window regulator. Unbolt those and you can pull the window up and wedge it out of the way. Or if you are talking about a sedan (4-door) you can just tape it up to the door frame. ie. a couple of peices of duct tape going up and over the frame of the door.

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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.