No matter where the cap is, it stops arcing or other noise. To trouble shoot, just disconnect it. If it is for the points, the car will then start if it was bad and you took it out of the circuit. Get a new one and put it in. It will run okay without it but pitting of the points will start so do not do it keep it out of the circuit long. If it is the dash capacitor, just replace it and be done with it. Capacitors only fail one of two ways: it shorts out and carries whatever it is hooked to straight to ground. On points the car won't start. The second, the capacitor "Leaks" which means it no longer works as a capacitor and does not filter noise. You will hear its effect in your AM radio when that happens. And it also leads to one side of the points to pit and the other to build up metal throwing your dwell off.