If your car’s door lock is failing, it might be helpful to know that the door locks are essentially identical on the E23 and E24 models from 1977 through 1986. The lock cylinder fits into the door with a complex structure that involves a tunnel at the end of the lock cylinder. Into that tunnel, we fit a tiny ball bearing, and a spring, and another ball bearing. Then, we turn the cylnder slightly, to keep these three items from falling out. We fit a spring with two tabs around the cylinder, and finally we slide in a plate that keeps it from falling out of the door.