I have power windows on my 87, but I think the setup is the same as to the "stopper" or "bumper."  Mine was missing on the driver side but I matched what I still had on the passenger side with a rubber bumper and sheet metal screw I purchased from a hdw. store.  Bit of a bear getting a stubby screwdriver underneath to drive the screw into the hole, but it has worked great!  It will indeed stop your window just a bit before it hits absolute bottom.  Can't tell you the exact size, but about 3/8" length and diam.  You can try a few screw sizes by hand w/o the bumper until you find one that seems right.  You'll need a bumper whose screw head is recessed so the window will hit rubber and not screw.  BTW while your door is opened up, look at the side of the window glass near the door latch to make sure the fabric pad is in place to keep the glass from rocking as it rises and falls.  There's a small metal "frame" fastened (welded?) to the inside of the window frame to hold this pad.  I bought a thick (1/4"?) self-adhesive furniture pad to stick on where mine was missing.

Jim Creech
87 Westy GL, 2.1

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  there is supposed to be a stopper at the base of the window regulator, I would appreciate it, as I hadn't the time or patience ( I was freezing!) last night to take the passenger door back apart to check for myself.  Also, if you can tell what form that stopper takes, it would be most appreciated.  Of course any other advice in this matter would be helpful too.

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