If you are refering to the door check strap then it is a very easy DIY.
there is a detailed description with photos on the DIY forum here for the 124 car. The 126 is similar in principal - First remove the door panel.If you are not familiar with that procedure - check the archives.Plenty there .Once you removed the panel, at the Pillar ( bottom of door) there is a pin with a circlip that holds the metal strap to the frame - remove. Next ,when the strap itself is free from the pillar remove two bolts,top and bottom (8/10mm or so) from within the rubber seal on the "outside " of the door frame.It is not always easy to see because the door does not open wide enough, but you can feel them or use a small mirror if you prefer. Last, reach inside from within the door frame cavity -(window up) there is one last bolt (same size) holding the strap to the door frame. Now just wiggle the check strap out.
The part is less than $20, check fastlane.
Makes a big difference in the way the door shuts.