I did that job on the drivers front window on my 126 a few months ago. You definitely need to remove the whole motor/regulator assembly. THe new plastic piece needs to be separated in half, and then the rivet is inserted and hammered out and open to keep it in the regulator arm. TO do this, you need to support the base of the rivet while you are hammering, or you will break the plastic piece and have to reorder and start again. I used a stack of coins. THe drilling out of the first rivet wasn't too bad with a fairly powerful, hand held drill and a new bit. The rivet pounding takes some time to get flat and spread the right amount. Good luck, this is a DIY job, no problem!