Start by checking the passenger rear door vacuum line.

The reason I say this is becuase in a (W201) the window regulator in that particular door is the one that goes out most often. If someone swapped it out, they may have unseated or incoorectly reattached the yellow vac line going to the actuator.

Other than that you will have to systematically check each connection.

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1994 E500 (MY SUNDAY DRIVER)
1993 190E SPORTLINE LE W/ M104 SWAP
1997 C230
2002 ML320
2000 BMW 528I (WIFEY'S CAR)

"Excuses are crutches for the unfounded."