When I bought it my 300CD would raise the passenger window 1/2way before it would stop then I'd hear a click-click-click-click-click. Turned out to be broken teeth on the window regulator arm.

You'll need to pull the regulator/motor to be able to see where the problem lies.

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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.