When I bought a used 1970 BMW 2002, the clock did not operate. Here's how I was able to repair the clock.
Removing the panel to the left of the glovebox, I reached up behind the clock and unscrewed the knobs holding it in place. Note where the wires go and remove them. The clock is mechanical with an electric motor to rewind it. As the clock winds down and a set of points make contact, current flows through the motor, rewinding the clock and resetting the points. I merely filed the points clean again and the clock became operative.
Your inoperative clock might have other problems but this solved mine.
Author: Richard L. Jones