When I bought the van the previous owner told me that the driver side window was stuck up and he had disconnected the switch and it needed a new plastic tab.
After doing research on here I found that tab is hard to get so went with a method recommended by jmoller99 on Sprinter-Source.com in this thread of making a aluminium tab.
Based on his idea I got a piece of aluminum I had lying around (1/8" by 3/4"), Cut it and ground it into shape. Cut out a slot in the middle and sanded it down really smooth. Then fit it into the window guide, attached it to the arm and made sure to put a ton of grease in there. Not sure what grease I used... either some bicycle grease or hub grease I had lying around. 2-3 years later it is still working great and required no permanent alterations to the arm pin.
Tools Used:
-Bench Grinder
-Grease (Brake Grease or similar)
-Aluminum Piece (1/8" by 3/4")
If you don't want to grind down the aluminum there are a few companies out there that make a brass replacement part for the original plastic slider that should last longer (here is one from Ebay).