Plastic slider or sometimes called a "jaw". Might as well buy 4 of them. The others will fail soon. There are two different styles. Remove a door panel to find which you have.
The first one you do will be a puzzle but after you learn the easy ways the rest are smooth.
I find getting the stinking door panels correctly back in place to be the hardest part of the job.
You have to pull the window regulator out (you'll need to do some finagling to get this out, do NOT pry or use force as you'll tweak the arms creating another problem), grind the back side of the jaw, remove the metal pin, insert the new jaw pin and "peen" the soft rivet to spread it and then re-install the regulator, panel and switches. The plastic slide of the jaw itself can be removed to ease the "peening" process. I find using a chisel to "X" the back side of the rivet works best.
Stock up on band-aids and minor curse words 'cause you're going to scrape your hands up on the first one you do.
Now's a good time to vacuum out the bottom of the door and check for rust. This is a problematic area for rust to begin on the W126's. If you do find rust then look at your window seals. If they are hard and cracked then you know where the water is entering. If there is rust, clean it, treat it and paint it now while you are there. Replace the draft excluder plastic if it tears.
Go over the door cards, clean and tighten the screws and clips.
Gee, ain't owning an older car fun?
