My drivers side belt was like that when I forst got the car. All I did was switch it with one or the backseat ones. I hardly ever take passengers and if I do, the seat belt back there works fine, they just gotta fish for it. I unbolted the seatbelt from the floor and it was, however, a little tricky to get the metal bracket through the plastic slot. It IS possible, just be careful not to break the new top plastic piece or go through all the trouble and put it on backwards so the belt latch hangs upside down or backwards when not in use.

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1982 300D Turbo "Helga"
380,000 miles