Battery isolator reminder for classic car
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Battery isolator reminder.

If I understand your question properly this circuit will do what you require.


If the isolator is left switched on and the ignition switch opened current will flow around the ignition switch, through R1/D1 and the buzzer and find its way back to the chassis via whatever load is on the ignition circuit. If there is no load then there will be no warning but this shouldn't matter as the battery won't be discharging.
When the ignition switch is closed the warning circuit is shorted out.


The buzzer is optional. The resistor is required to limit the current through the LED to about 10 mA.