The Car...1988 300SEL (W126 Chassis)

I'm trying to replace the front passenger window regulator. I have had the regulator in and out three or four times. Every time I try raise the window with new regulator installed it keeps binding up after a couple of inches of movement and stops moving. After so more poking around I found the part pictured wedged in at the top of the window, not really connected to anything. Can someone tell me what this part is and how it gets installed...

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