Some younger classics have connector blocks on the ends of the branches of their cable harnesses. And those multi-connectors are not all in good visibility.
However, they can suffer from wetness. splash water and penetrated sludge. For example, if the dashboard of your classic car is going to do incomprehensible things, then it is wise to go hunting for multi-connector blocks that may or may not be hidden.
Unloading and attaching a few times with a shot of WD 40 or contact spray can do wonders.