The majority of clock failures is due to the choice of main power regulator. THE clock circuit runs on 5v. THE 3 most practical ways to get 5v from 12 are switching regulator, pass transistor regulator, zener regulator. These are listed by order , highest to lowest. Cost, efficiency, complexity.
They chose the cheapest simplest and least efficient one. SO it has to cook away a watt or 2 of heat. In a small enclosed box. THAT is installed in a closed space, dashboard, where the ambient temperature can get over 140 degrees. The resistor that drops most of the voltage as waste heat gets so hot that it causes the solder on its leads to corrode and the connections fail. A linear pass transistor regulator would have to dissipate roughly half the power that the zener regulator does. Even back then, those were cheap and common. Side note, a switcher was not a financially practical choice back then.
There is one other feature of the clock design that can be improved also. BY adding a diode and capacitor to the keep alive power feed, the clock doesn't forget the time when starting the car like it did a few times per year, before I added them. They actually had no filter on that line to smooth over momentary voltage dips.